TRANSACTIONS OF SECTIOX A. G10 



.?' inches in wicltb, were placed side by side, touching one another. The rays 

 from the two outside lenses were reflected 90° outwards, and when the necessary 

 interval required between the pictures had been traversed the rays were again 

 reflected 90^, and so all three images fell on one long plate. In the first camera 

 I made, a second lens for the side images was placed in the path of the outer 

 rays and came to a focus on the plate. They were so chosen that the images 

 were of the same size as the central image. I may say that the results I got 

 with this camera were exceedingly good, and all last winter I used it for ray 

 three-colour photographs. It had, however, the disadvantage that it was a 

 camera with a fixed focus, since if the plate were moved back from the focus for 

 parallel rays the central and side images no longer remained equal. I worked 

 out a new optical arrangement, which had the advantage that all three images 

 increased equally as the plate was moved from the focus for parallel rays. The 

 problem that presented itself was this : to find combinations which gave images 

 of equal focal length for parallel rays, and at the same time that the second 

 principal points were at the same distance from the front lenses. If this were 

 solved, as the three lenses were always at the same distance from any object, 

 however near, a movement of the plate from the front lenses would still give 

 images of the same size, no matter what distance the focus might be away. 



As the problem also had to take into account the interposition of mirrors, it 

 required many trial calculations to get the most suitable foi'm of combinations 

 which would keep the camera of reasonable dimensions. Eventually I fixed on 

 three combinations which answered my purpose. The two outside combinations 

 were of the same form, but the centre one was totally different. Beginning with 

 the centre combination, the front lens was a convex of some 9-inch focus, and 

 a second convex lens was placed about 4 inches away, of such focal length as 

 to give a focus of 6 inches, the plate being placed 7'5 inches from the front 

 lens. In the two side combinations the small front lens is concave, and the 

 second lenses are placed about 15 inch aAvay from tlu^ first lens. The 

 first mirrors are not interfered with ; the}" send the rays through the second lens 

 and these fall on the second mirror. The images given by these three combina- 

 tions are of the same size, each principal focal distance being the same. Further, 

 the second principal points are at the same distance from the front lenses, and thus 

 the problem was solved. I have brought the new camera with me. The first 

 difficulty that I had was as to the material of which to make the mirrors. 

 Having noted that a mirror made for Mr. V. Boys for some of his experiments 

 was made of steel, and that after a lapse of many years it was pi-actically unaltered 

 in brightness and had no rust about it, I decided to make my mirrors of steel. 

 They are readily worked to a flat sufficiently good for my purpose at any rate, 

 and are supposed to reflect 60 per cent, of the incident light. I have not 

 found it quite so much, 50 per cent, being nearer the mark. As there are two 

 reflections I only got 25 per cent, of the light which strikes the first mirror. 

 This, however, may be due to the fact that in order to render the steel surface 

 impervious to damp I have coated all the mirrors with a very thin coating of 

 celloidin varnish, which preserves it wonderfully (as it does that of silver, I may 

 add), and does not alter the reflective power to any great extent ; nor does it 

 cause any distortion. As a second precaution against damp, chloride of calcium 

 is kept in the camera, in the same form as used for preserving platinum paper. 



It may be asked. What about distortion of the image and flatness of field and 

 correction ? The lenses are simple lenses and not achromatic. Correction for 

 colour is unnecessary, since only a small portion of the spectrum is traversed on 

 any one of the three screens. The flatness of field, Mr. Dennis Taylor, who 

 has kindly given me every help in making the lenses for me, secured by dividing 

 the second side-lenses into a pair and making them a compound lens. He also 

 corrected the curvatures of the lenses, for the three different colours. There is a 

 small quantity of distortion in each image, but the distortions are all equal. 

 The cfl,mera itself, with its mirror and fittings, was cari-ied out by Mr. Colebrook, 

 of the Royal College of Science, under my own eye. I am greatly indebted to 

 him for the care he has taken and the high-class work he has put into it. 



