118 PROCEEDINGS OF THE SOCIETY. 



wholly of one material, viz., fused quartz. That Dr. M. von Rohr 

 should have been able to compute Microscope objectives free from 

 spherical aberration from one material only was a remarkable achieve- 

 ment. 



The photographs shown included a set of four of Amphipleura 

 pellucida, taken under varying conditions with a 1 • 7 mm. monochromatic 

 objective of N.A. 1*25, using light from the cadmium spark. The 

 resolving power of this objective was therefore as great as that which 

 an objective used with ordinary white light would have were it possible 

 to give it a N.A. of 2*5. One of these photographs of Amphipleura 

 pellucida, taken with oblique light from the condenser, showed the 

 diatom clearly resolved into dots. 



There were also five photographs of Surirella gemma taken with the 

 same objective, and attention was called to the beautiful crispness and 

 sharpness of the images, and also to the way in which the black and 

 white dot effect came out. 



Along with the photographs were comparison photographs of the 

 same diatoms, giving the same magnification, viz., about 1800 diameters, 

 taken with a 2 mm. apochromatic objective of N.A. 1'40, using light 

 from a magnesium spark (A = 383 /a/a). The difference in the appear- 

 ance of the images was very apparent. 



Mr. Rheinberg remarked that the photographs showed that diatoms 

 behaved in the ultra-violet light of the cadmium spark much as in 

 ordinary light, but pointed out that many other substances transparent 

 to ordinary light were more or less opaque to the light in question. 

 For instance, it had been found that the fibres of the crystalline lens of 

 the eye, certain cell membranes of plants, etc., were opaque to this 

 light, and therefore yielded totally different images, which in some cases 

 were clearer. 



Taking this into account, besides the increased resolving power, he 

 thought the new method of photomicrography was not only one of the 

 most interesting, but also one of the most, if not the most notable ad- 

 vance in practical microscopy of modern times, and one, moreover, which 

 might be very far-reaching in its consequences, as the first step having 

 now been successfully accomplished as regards the use of monochromatic 

 light of shorter wave-length than the ordinary, the future held the 

 possibility of continuous gradual progress in this direction, accompanied 

 by greater and greater resolving power. 



The President thought that what Mr. Rheinberg had described was 

 certainly a very important advance in photomicrography. 



Mr. C. L. Curties said that the photograph of Amphipleura resolved 

 into dots was one of the finest yet shown, but this was not the first time 

 this diatom had been so resolved, for he remembered that a photograph 

 showing the dotted structure was made by Mr. Gifford, and also that 

 Dr. Spitta not only exhibited a photograph, but also the diatom itself 

 in the adjoining room. They were, of course, not so distinctly resolved 

 as the specimens now exhibited by Mr. Rheinberg. 



Mr. Rheinberg said that he had not been aware that A. pellucida 

 had already been resolved into dots. Whether new detail were seen, or 

 whether merely an increased crispness were visible in the image, depended 

 on the nature and degree of fineness of the structure being viewed. 



