Charles Jasper Joly

“ Closing one eye and looking vertically downwards with the other through a slip of plain glass, e.g. a microscope cover-glass, held close to the eye and inclined at an angle of 45° to the horizon, one can see the images of objects in front, formed by reflection from the surface of the glass, and at the same time one can also see through the transparent glass. ”

“ It is desirable for two reasons that the image should lie in the plane of the paper, and this can be secured by placing a suitable lens between the object and the prism. If the image does not lie in the plane of the paper, it is impossible to see it and the pencil-point clearly at the same time. Moreover, any slight movement of the head will cause the image to appear to move relatively to the paper, and will render it difficult to obtain an accurate drawing. ”

“ The brightness of the image is sometimes increased by silvering the glass; and on removing a small portion of the silver the observer can see the image with part of the pupil while he sees the paper through the unsilvered aperture with the remaining part. ”