J. S. Plaskett

“ In essence a spectrograph consists of a narrow slit, one or two-thousandths of an inch wide, on which the star light is focussed. That passing through the slit falls on the collimator lens which makes it parallel and then on a prism or prisms, triangular shaped pieces of glass which change the direction of the light and decompose it, breaking it up into its constituent rainbow colours. The spectrum, as it is called, is focussed by a camera lens on a photographic plate or can be viewed by a small telescope if desired. ”

“ It has been truly said that the degree of civilization of a country may be judged by the support it gives to the study [37] of astronomy. By the establishment and maintenance of the Dominion Observatory at Ottawa and of the Dominion Astrophysical Observatory at Victoria with the second largest telescope in the world, Canada has a just claim on this criterion to the favourable estimation of the scientific world. ”

“ With the one-prism spectrograph a speed of one mile per second means a displacement of the lines of one thirty-thousandth of an inch. Thus if the lines are shifted to the violet by a thousandth of an inch, the star is approaching the earth with a speed of 30 miles a second. These displacements are accurately measured by a microscope and the measurement of the radial velocities of the stars is one of the main researches of this observatory. Obviously with such small displacements to be measured the greatest [30] care must be taken to avoid all sources of error. ”

“ As already indicated most of the work with the 72-inch telescope is spectroscopic, but as also indicated modern spectroscopic investigations cover so wide a range of research that the actual work of the observatory is very varied. By aid of suitable spectrographs attached to a large telescope we can measure the speed of the stars towards or from us, their radial velocity as it is termed. We can discover double stars too close ever to be seen double in any telescope and we can determine the manner in which they revolve around one another and their distance apart and mass. ”

“ A third misconception is that the astronomer only works at night. However true this idea may have been in the days of visual observations when the measurements were made at the eyepiece, there is certainly now, when photography is so generally applied, more day than night work in astronomy. Besides the advantages of permanency, accuracy of measurement and power of recording objects beyond the range of the keenest eyesight, the photographic method has the further great advantage that an hour’s exposure may give sufficient material for several days’ measurement and discussion. ”

“ The main superiority of modern over ancient civilization does not consist in the greater abundance of the necessities and luxuries of life, although this is undoubtedly due primarily to scientific research, but to the elevating influences of the truer conceptions of nature made possible by the abstract study of astronomy and other sciences. ”

“ The refracting telescope is the most familiar type as the ordinary spyglass or draw-tube telescope and the field or opera glass are all refracting telescopes. The refracting telescope is so called because the light from the distant object is refracted through a lens at the outer end of the tube and forms an image of the object at the inner end, just as a camera forms an image on the ground glass or film, and this image is viewed and magnified by the eyepiece or ocular. ”

“ Modern research on conditions in the stars has made it practically certain that the enormous supply of energy, which has been radiated into space for aeons of time from these bodies, can only be maintained undiminished by the energy released by the transformation of atoms in the interior of the stars, where conditions of temperature and pressure prevail at present unattainable in terrestrial laboratories. ”

“ A description of the method of setting upon the required star, when, for example, photographing the spectra of the stars, will help to make the operation of the telescope more clearly understood. It is easily possible to pull the telescope around by hand to the required star identified by eye among the constellations. Although the moving parts of the telescope weigh nearly 45 tons, so perfect are the ball bearings in which it turns that a weight of 3 pounds at the upper end of the tube is sufficient to set it in motion. ”

“ The spectrographs of the 72-inch telescope have unequalled defining power and are the last word in convenience of manipulation and accuracy of work.
Owing to the faintness of the star light and to its being spread out into a spectrum a considerable time is required to photograph the spectrum of a star, about 20 minutes for the sixth magnitude, the limit of visibility to the unaided eye, when photographed with one prism, while three prisms will take nearly five times as long. ”

“ It is evident, by rotation of the telescope on the declination and polar axes by means of the quick-motion motors, that the tube can be pointed in any direction in the sky, towards any star. But owing to the rotation of the earth on its axis from west to east, which is the cause of the apparent motion of sun, moon and stars from east to west, the telescope will be quickly carried eastward of the star which will only remain for an instant in the field.
The mechanism by which the rotation of the earth is compensated for is called the driving clock and is contained in the case L, Fig. ”

“ The spectrograph must be exceptionally rigid to avoid differential bending as it moves with the telescope. As change of temperature can produce spurious shifts of the lines, the temperature must be kept as constant as possible, this being effected here by a very accurate electrical thermostatic device called the Calendar Recorder which maintains the temperature constant to one-hundredth of a degree. The optical parts must be of the highest quality to give perfect definition to the spectrum lines and many other precautions must be taken if accurate work is desired. ”

“ A new method depending upon the use of a wedge of dark glass has been applied to determining the distribution of energy in the different parts of the spectrum of the stars and to measuring their temperatures, while an application of the same methods to individual lines may lead to a great increase in our knowledge of conditions in the stars. ”

“ Poincaré has well said that if the earth had been so continuously covered with clouds that the heavenly bodies could not be seen, mankind would still be in a primitive state and under the domain of superstition. ”

“ But it is now generally recognized by the layman as well as the scientist that without abstract there can be no applied science and that all the great economic and industrial applications of science have had to be preceded by the abstract and apparently non-practical investigations of pure science. ”

“ On the other hand the reflector, on account of its perfect achromatism, is the instrument par excellence for photographic observations, and, as more than three-fourths of modern astronomical work is photographic, it appears to be superseding the refractor. ”

“ It should not rise above the shade temperature during the day and should rapidly assume and follow the external temperature at night. Such materials as brick or stone are obviously not suitable and all recent telescope buildings are entirely of metallic construction in order to assume quickly the night temperature, and of double-walled, ventilated type to prevent overheating from the sun’s rays. The building for the 72-inch telescope is entirely of steel construction, circular in form, 66 feet in external diameter and with vertical walls 32 feet high. ”

“ From the spectra of the stars we can determine their absolute brightness as compared with the sun and their parallax or distance. The chemical elements present in the outer atmospheres of the stars can be determined and the pressure in these atmospheres. ”

“ The lower, south, bearing is carried in a massive cast iron pedestal bolted to the south cement pier. The polar axis is rotated on these bearings, also at the rate of 45 degrees per minute, carrying the declination axis and tube with it to any position east or west in the sky by an electric motor and reduction gearing concealed within the south pedestal. The position east or west in the sky, the right ascension as it is called corresponding to longitude on the earth, is read by means of a graduated circle shown above G, Fig. ”

“ These with the three motors operating the dome are all continuous current motors which can be started and reversed more readily and have greater initial torque than alternating motors. Each motor is supplied with an automatic control, so that all that is necessary is to throw the switch or press the button to start or reverse. Current is supplied by a motor generator set on the ground floor. ”