234 SUMMAKY OF CURRENT RESEARCHES RELATING TO 



The mathematical principle involved is that the interocular distance, 

 normally 62 mm., has to be divided by the required magnification in 

 order to give the angular separation through which the objective must 

 be moved. Thus, for 32 diameters, the separation would be 2 mm. ; 

 and for 1000 diameters, 62 micra. In practice the photographic 

 objective is kept still, and the object moved. The author uses two of 

 Zeiss' optical benches, mounted on trestles. For very low magnifications 

 (to about x 10) one only is used ; for higher magnifications they are 

 placed end to end. At one end of the bench, fig. 31 (pi. III.), is fixed the 

 lamp casing, the bench itself carrying the condensers, object-stage, lens 

 and camera, all of them adjustable in any position on the bench. The 

 camera itself is a very simple affair, adapted for the English standard 

 stereoscopic size, 6f X 34 inches. The formula regarding objective 

 separation resolves itself in practice ■ into two parallel lines drawn on 

 the focusing-screen, 62 mm. apart, by means of the stage. The object 

 is moved until one of the lines cuts the image centrally : the first 

 exposure is then made ; the object is next transferred to the other line, 

 when a second exposure will give the truly stereoscopic pair. The 

 author uses Zeiss' " planar " photo-objectives, their very flat field making 

 them especially suitable for this work ; their aperture is, however, too 

 low for high magnifications. Incident light seems to be more satis- 

 factory than transmitted light. The Nernst electric lamp, with a 

 one-ampere filament, makes an excellent light-source. Incandescent 

 gas is also good, but requires long exposures. It is essential that each 

 picture should have identical exposure. Arc-light involves a risk of 

 burning a specimen, but only requires short exposures ; it is a great 

 help when dealing with autochromes, which have, however, special 

 difficulties. 



In the figure, B parallelises the rays ; T) is a long-focus lens for 

 converging the parallel rays, after reflexion from mirror H, on the 

 object ; E is a short-focus lens ; F a plano-concave lens to parallelise 

 the converging rays from E (this gives a parallel beam of small diameter, 

 but of great intensity) ; G, object-stage, laterally adjustable by means of 



EXPLANATION OF PLATE III. 



A. Lamp casing containing hand-feed arc lamp. 



B. Lens to parallelise rays from arc. 



C. Water-cooling chamber. 



D. Long-focus lens, converging the parallel rays, after reflecting from mirror H, 



on the object 0. 



E. Short-focus lens. 



F. Plano-concave lens to parallelise the converging rays from E. This gives a 



parallel beam of small diameter, but of great intensity. 



G. Object stage laterally adjustable by means of the vertical pinion. 

 H. Small mirror universally adjustable. 



I. 35 mm. lens (Zeiss Planar). 



J. Focusing pinion. 



K. Camera. 



L. Optical bench, on which the whole of the above is adjustable. The optical 

 axis of the condensing system is 52 mm. above that of the camera. The 

 horizontal line shows the course of the central ray of light. The con- 

 densers D and E are mounted on a hinged fitting, the one not in use 

 being folded down out of the path of the rays. 



