Optical disc

“ In each sector the same scene is represented, with this difference, that the movements of the objects are so arranged as to be progressive from one extreme to the other. The disc being fastened to the pin a b (fig. 3) by the screw v, with the figures facing outwards towards a, the whole apparatus is held before a looking-glass by the handle m. If the disc be now rotated by the button b, and the eye placed opposite one of the square holes in the card, the figures on the disc will appear to move more or less quickly according to the rate at which it is rotated. ”

“ Those who have the means and perseverance to make their own mirror, should be careful not to proceed with the finishing touches until an hour at night when their workshop, if in a town, is free from tremor.
First secure a well-ground and carefully-centred disc, let the polish be as perfect as possible before any attempt is made to figure. Care must be taken that every square or portion of the polisher is of the same consistency and temperature, that the disc may not be acted on irregularly. ”

“ With regard to object-glasses, those of shortest focal length have the highest powers, and the longest eye-pieces have the lowest powers. The body of a microscope, or principal tube of which it is composed, is, in the best instruments, about nine inches long, and a draw tube, capable of being extended six inches more, is frequently useful. From simple optical principles, the longer the tube the higher the power obtained with the same object-glass; but only object-glasses of very perfect construction will bear the enlargement of their own imperfections, which results from the use of long tubes ”

“ The complete testing of glass for uniformity of refractive index can only be arrived at by grinding and polishing a sufficient portion of the surfaces to enable an examination to be made of every part. In the case of a small disc it is sufficient to polish two or three facets on the edge, and to examine the glass in a field of uniform illumination through the windows thus formed. ”

“ In order to render the apparatus capable of transmitting different signals, two similar discs, on each of which was marked a number of words, letters, and figures, were attached to the seconds-arbors of two clocks beating dead seconds, and the discs were thus made to rotate synchronously before the operators at the two ends of the line. In front of either of these rotating discs was placed a fixed disc, perforated at one place, so that only one symbol was visible at a given time to either operator. ”

“ A disc of lead or iron, of the same diameter as the glass, and of approximately the proper curvature, is prepared by turning, and is armed with a handle projecting coaxially from the back of the disc. The glass revolving with moderate speed on the lathe, the lead tool, supplied with coarse emery and water, is held against it, care being taken to rotate the tool by the handle, and also to move it backwards and forwards across the disc, through a distance, say, up to half an inch; if it is allowed to overhang too much the edges of the glass disc will be overground. ”

“ The prismatic ring section is ground into the face of a glass disc, and from one end to a point half around it has its base outward, and from this midway point around to the other end having its base inward. The warp from one end to the other is gradual.
A beam of light passing through this ring, in rotation, is caused to oscillate, having its hinged action fulcrumed in the plane of rotation of the prism ring. The oscillation is always in the plane of the diameter of the disc from the point where the light passes through the prismatic ring section. ”

“ Dollond had rediscovered in 1758 the method of counteracting colour in glass lenses, but no one seemed disposed to apply the principle on a large scale. Apparently the way here also was barred against progress. All attempts to manufacture discs of pure flint glass larger than seven inches in diameter failed. Up to that point the achromatic refracting telescope was a great success. For seventy years good specimens of considerable size were exceedingly rare, and even in 1830 a disc of eleven inches and seven-tenths in diameter cost £1000. ”

“ A disc of card five inches in diameter, with six equidistant holes half an inch from the edge, answers well. The disc must be made to spin by any means very regularly at such a speed that the tuning-fork, or stretched string if this be used, when looked at through the holes, appears quiet, or nearly quiet, when made to vibrate. The separate drops will then be seen, and everything described in the preceding pages, and a great deal more, will be evident. This is one of the most fascinating [Pg 161] experiments, and it is well worth while to make an effort to succeed. ”

“ I did this by hand labour with the aid of emery, but without a lathe. I finally used rouge instead of emery in grinding down the glass, until I could see my face in the mirror quite plain. I then sent the 8 3/16 inch disc to Mr. George Calver, of Chelmsford, to turn my spherical curve to a parabolic curve, and to silver the mirror, for which I paid him 5L. I mounted this in my timber tube; the focus was ten feet. When everything was complete I tried my instrument on the sky, and found it to have good defining power. The diameter of the other glass I have made is a little under six inches. ”

“ Upon the disc is mounted a tiny mirror, and as the suspension of the disc is in the magnetic field held there by the strength of the field itself, it is extremely easily disturbed, so that a small beam of light reflected from the mirror can be vibrated with a very little current through great amplitude. ”

“ The loss of light in its journey from the illuminant to our eyes is enormous; the disc on the screen is simply a greatly magnified image of the light, and here is great loss; add to this the interception of some light by the opacity of the slide, and the fact that much more is lost in reflection from the screen, and absorption by the screen, and it is easy to realize that the image from the screen reaching our eyes is vastly less bright than that reaching the eye when, for instance, we examine a slide in the hand by transmitted light. ”

“ If time is no object, we have an easy means of still further increasing the magnification. To do so we cut a thin sheet of copper in such a manner that it just fits our lens tube, in front of the diaphragm. Then in the very centre of the copper disc we make a hole with a “number one” needle, the hole is about one twentieth of an inch in diameter. Replace the convex lens in its cardboard holder and screw on the front portion of the camera lens. ”

“ Glass manufacturers seem to experience unusual difficulties in their attempts to form large discs of optical glass pure enough and uniform enough to be suitable for telescopes. These difficulties are enhanced with every increase in the size of the discs, so that the cost has a tendency to increase at a very much greater rate. It may be mentioned in illustration that the price paid for the object-glass of the Lick telescope exceeded ten thousand pounds. ”

“ When this disc is caused to rotate rapidly, we shall have first a patch formed by the light coming through one slit, and then another formed by that coming through the other slit, thrown on the screen on the same place in rapid succession, and the effect on the eye should be precisely the same as if the disc was not there, save in the matter of intensity. ”

“ The margin of the disc will be seen to be notably less bright than its central portions; and near the margin, and oftenest, though not invariably, in connection with one of the dark [pg 50] spots, there will be markings of a brilliant white, and often of a fantastically branched shape, which seem elevated above the general surface; while as the eye becomes more used to its work it will be found that even a small telescope brings out a kind of mottled or curdled appearance over the whole disc. ”

“ It is so difficult to procure large discs of flint glass for optical purposes, to produce the requisite curves of the different lenses, and to combine them together with that extreme accuracy which is requisite, that when a good compound lens of this description is found perfectly achromatic, the optician must necessarily set a high value upon it; since it may happen that he may have finished half a dozen before he has got one that is nearly perfect. ”

“ The brighter the colour the more rapid it should be. We learn from this that the diminution of the more intense impressions on the retina is more rapid at first than of the feebler.
Fig. 41.—Electro-motor with Discs attached.
Fig. 42.—Method of cutting Disc to allow an overlap of a second Disc.
Very convenient discs for producing colours by rotation of sectors may be made by the following: vermilion (V) , emerald green (E) , French ultramarine blue (U) , chrome yellow (Y) , lamp-black (X) , and (zinc) white (W) . With these nearly every colour can be produced, or its value derived. ”

“ Having levelled the instrument, so as to render the disc concentric with the plate, and placed the index at zero, it is obvious that if an electric charge through the wire, h, reach the plate with the arms, it will electrify [Pg 127] the disc and index: the disc will have the opposite electricity, and the extremities of the index will take the same electricity as the arms, and consequently the index will describe an arc more or less great. The motion of the index is sufficiently slow to allow the eye conveniently to follow it. ”

“ If you throw on the screen a circle of light, coloured by passing the rays from the electric lamp through chloride of chromium, the disc will then consist chiefly of red rays mixed with the rays which are absorbed by the yellow spot. If the observers in the darkened theatre look at the disc and wink slowly, they will see, with more or less distinctness, an illusion like red clouds floating over the disc, in consequence of most of the rays other than the red being absorbed by the yellow spots in their eyes. ”

“ Years ago, there was a little instrument of a similar kind, sold for a penny in the streets of London, which consisted of a morsel of looking-glass set at an angle, in a pill-box, and which gave the possessor the power of seeing all that was going on behind him. Persons who wear dark preservers are often in the habit of observing all that is going on behind their backs by the reflection seen in the corner of their glasses.
Such are the principal optical recreations founded on the reflecting and refracting properties of mirrors and lenses. ”

“ When the philosopher desires to apply glass in the construction of perfect instruments, and especially the achromatic telescope, its manufacture is found liable to imperfections so important and so difficult to avoid, that science is frequently stopped in her progress by them—a fact fully proved by the circumstance that Mr. Dollond, one of our first opticians, has not been able to obtain a disc of flint glass 41⁄2 inches in diameter, fit for a telescope, within the last five years ”

“ Any number of such discs can be combined so that each shows a sector at the top, and the angle of each sector can be varied at will by sliding the corresponding disc between the others. Maxwell used discs of two different sizes, the small discs being placed above the larger on the same pivot, so that one set forked a central circle and the other set a ring surrounding it. ”

“ The vibrations of the voice or of music, which reach the disc, cause this needle or point to rise or fall, producing impressions upon the tin foil or other substance. After the record has been made, duplicates are produced in a substance largely made of rubber, which is placed on a rotary disc or cylinder that is turned automatically, the needle or point attached to the disc working into the grooves and rising with or following the impressions, which cause the plate or disc to vibrate. The process is wholly mechanical, and electricity is not used. ”

“ It is common for such lights to be seen at a distance of between fifty and sixty miles. The apparatus is mostly arranged in the form of an octagon, and is generally provided with additional reflecting mirrors at those parts above the light which are out of the range of the lenses. The light shining fully in eight directions at one time, can scarcely be missed by any ship within range; but in order to guard against any possibility of accident, the optical apparatus is often made to revolve by clockwork, so that every point of the ocean is illuminated in turn. ”

“ The central disc of the lens, marked B in the accompanying diagram—as employed in lights of the first order—is about 11 inches in diameter, and the focus distance equals 920 millimètres, or 36.22 inches. The annular rings surrounding it gradually decrease in breadth as they recede from the centre, from 23⁄4 to 11⁄4 inches. The lens, we should add, is made of crown glass. ”

“ If, howover, the arrangement is slightly varied, so that transmitted instead of reflected light is made use of, comparatively feeble illumination is sufficient. A very effective way is to turn a small metal [Pg 190] disk, having an open sector of about 60°, in front of a sheet of ground or opal glass behind which is a lamp. By an arrangement of this kind upon a larger scale, the effect may easily be rendered visible to an audience. The eyes should not be allowed to follow the disk in its rotation, but should be directed steadily upon the centre. ”

“ The round optic disc is represented near the centre of the figure. At its outer and inner margin it is skirted by a crescentic patch of brown pigment (an accumulation of pigment granules level with the hexagonal cells) ; this is sometimes observed in healthy eyes. The middle of the optic disc has a whitish, the rest a brilliant pink colour (the disc in the figure is too red) . The whitish portion is solely occupied by opaque white connective tissue, which surrounds the blood-vessels where they pass through the optic disc. The bundles of optic nerve fibres occupy the pink portion of the disc. ”