464 Transactions. — MisceUancous. 



same eyepiece involved the necessity of a greater length of 

 telescopic tube in front of the camera. I found, however, that 

 the increased scale of the objective, due to the flat curvatures 

 of the lens, enabled me to use an eyepiece of much lower power 

 for obtaining a picture of the same size and scale as formerly ; 

 and that, to effect this, the wliole optical combination need not 

 exceed Sin. in length. 



To overcome the chromatic and spherical aberrations above 

 mentioned, I constructed a back combination consisting of a 

 convexo-concave single lens, in near contact with the concave 

 side of which a plano-convex achroinatic lens was placed. 

 This, though somewhat reducing the scale of the picture and 

 the area of the circle of light, rendered the instrument nearly 

 non-chromatic and perfectly rectilinear. I could now produce 

 photographs the scale of which exceeded that of an ordinary 

 half-plate lens, including an angle of 40°, five, ten, twenty, or 

 even thirty diameters, simply by giving to the back lens a 

 greater power of dispersion. 



Used microscopically, I found that I could cop}' objects 

 size for size at distances of 10ft. or 20ft., or more, thus ad- 

 mitting of the proper and effective lighting of the object to be 

 photographed, and, in the case of spherical, cylindrical, or 

 conic bodies, giving a representation more in accordance with 

 the distance at which most objects are viewed ; and I have 

 no doubt that the larger forms of the instrument will within 

 certain limits be very useful for microscopic work up to six- 

 teen or eighteen diameters. Most of the photographic prints 

 which I exhibit were taken with the 2^in. lens, having the 

 eyepiece or back combination last described. 



This instrument, although it did fairly good work, was on 

 the whole too bulky and heavy for use in the field, and I had 

 for field-work to devise a lighter and more handy form of the 

 same, which, with some modifications, is before yoiT. 



In this, the optical part consists of a Eoss's "rapid symme- 

 trical lens,'" which, with the tube attached, is fitted to a tra- 

 velling-screw, thus enabling the focussing of the instrument to 

 be effected, if it is not desired to effect this by shifting the 

 camera-screen. The outer photographic doublet being free 

 from both chromatic and spherical aberration, it is not neces- 

 sary to be so careful as respects the eyepiece, because aberra- 

 tion is not so likely to be present in that part of the instru- 

 ment. Therefore the eyepiece fitted in the after-piece of the tube 

 is a simple biconcave lens serving only the purpose of dispers- 

 ing the rays at a greater angle than that at which they escape 

 from the back combination of the rectilinear lens ; otherwise 

 it acts in the same way as the instruments I have described at 

 greater length, with this variation only : that, if the distance 

 between the outer combination and the eyepiece be increased 



