272 COEBESPONDENOE. 



eyes, they must see, and paint, stereoscopic effects. Did not one of the 

 m-ost able and learned of tbeni all announce the true stereoscopic theory 

 400 years ago, and practise its taachings, as all great painters have 

 done ever since? Why, then, seek to exclude it from photographic 

 portraiture ? 



In thus advocating a certain amount of stereoscopic effect, I would 

 limit it to that quantity under which people see one another; that is, to 

 the quantity due to a lens of 2^ inches diameter, that being the 

 average space between the pupils of our eyes, which I may be per- 

 mitted to call their " stereoscopic aperture." With such a lens, a head 

 would be reproduced as we see it in nature, without the hard, cutting, 

 lines of monocular vision, which all painters deprecate, and avoid. 



Dr. Pigott will excuse me passing over a variety of subjects intro- 

 duced into his letter : I confine myself entirely to a small branch of 

 the optics of photography. 



G. S. C. 



Object-glasses. 



To the Editor of the ^Monthly Microscopical Journal.^ 



Padnal Hall, Chadwell, Essex, March 4, 1872. 

 Sib, — In No. XXXI. of this Journal, for July, 1871, page 36, there 

 appeared a short notice from Mr. Tolles describing an experiment 

 intended to prove that a pencil of rays of more than 82° can pass from 

 a balsam-mounted object with a wet-front or immersion lens. This 

 has nearly escaped my recollection, and perhaps is forgotten by most 

 of your readers. In the succeeding Jom-nal, while giving Mr. Tolles 

 credit for truth and honesty in the measurement of the angle shown, 

 I pointed out how the error had arisen from a refraction caused by 

 the position of the under hemispherical lens. Knowing that it is no 

 easy task to bring this within the comprehension of minds not familiar 

 with optical science, I took the pains to show, by a direct experiment, 

 that the angle was considerably diminished by water. Canada balsam 

 might have been tried in the same way, but I objected to injure my 

 object-glasses by immersing their fronts in a tank of balsam merely 

 for the sake of demonstrating a result which I thought the most 

 obtuse could foresee. In water a dry-lens aperture of 170° was 

 reduced to 100°. Mr. Tolles seizes upon this to show that I am 

 wrong by my own exi^eriment on the point — that a greater ai)erture 

 than 82° cannot pass from a balsam-mounted object. Had the experi- 

 ment been repeated with the front immersed in a tank of balsam in 

 lieu of water, it needs no demonstration to convince those conversant 

 with the laws of refraction that the aperture would then have fallen to 

 below 82°, and further, though the aperture is 100° on an object in 

 the water, yet when that object is in balsam as usual, and under the 

 water, that the rays would again be refracted outwards, reducing 

 the angle from the object to within 82°. Mr. Tolles is unable to 

 perceive this, and so let it remain. Mr. Tolles has accoi)ted the only 



