Stereo-Photomicrographs. By H. Tavemer. 261 



except to mention that all the photographs on exhibition to-night 

 were taken with stops having circular apertures of 2^-4 mm. in 

 diameter, and with the inner edge of the aperture in no case more 

 than 1 mm. from the optic axis of the lens. It will be found 

 necessary to vary the size of the aperture in the stop according to 

 the thickness of the object to be photographed, and the objective 

 in use ; but the best results, without distortion or exaggerated 

 stereoscopic relief, are to be obtained by the use of a stop having — 



1. The largest possible aperture that will give sufficiently sharp 

 detail in the picture. 



2. The inner edge of the aperture as near the optic axis of the 

 lens as will give sufficient stereoscopic relief, and at the same time 

 the truest appreciation of the thickness of the object, or third 

 dimension. 



For convenience in working, Mr. Curties has made for me a 

 small piece of apparatus which screws into the nose-piece of the 



Fig. 38. Fig. 39. 



Microscope, and is itself of the nature of a secondary nose-piece, 

 to facilitate the changing of the stops. It is fitted with a swing-out 

 arm, carrying a ring which holds the stops, and which revolves 

 through 180° exactly, thus changing the aperture in the stop from 

 one side of the optic axis to the other, for the purpose of making 

 the two exposures. Except for this revolution of the stop, the two 

 photos are taken without any alteration whatever in focus, illu- 

 mination, position of object or apparatus, except, of course, the dry 

 plate. 



To obtain the two pictures on one plate in the correct position 

 for printing and mounting without cutting, I use what is known 

 as a repeating back, a specimen of which is exhibited here this 

 evening. One picture is taken at a time, that on the left half of 

 the plate, with the aperture in the stop to the left of the optic 

 axis, and that on the right half of the plate, with the aperture in 

 the stop to the right of the optic axis. The images are now on the 

 negative in the correct positions for printing and mounting, with- 



EXPLANATION OF PLATE X. 



Forarninifera (Orbulina universa), showing internal Globigerine structure, x 25. 

 Mounted by Mr. A. Earland. Photographed with a 2-5 rnna. stop. Beck's 

 1^-in. objective and No. 1 eye-piece. 



