1885.] NEW-YORK MICROSCOPICAL SOCIETY. 103 



He uses for this kind of work a Spencer yVth-inch lens. He sent 

 these pictures without expressing an opinion as to the value of 

 the medium. It is important to get a mounting material which 

 will give a good photograph. Balsam has been objected to. 

 The new medium, consisting, as it does, of a salt dissolved in 

 glycerine, has an advantage, since its refractive index can be 

 easily varied by varying the proportion of the ingredients. It 

 has also a disadvantage. It is not dense enough — especially 

 when, through dilution, the index is not higher than 1.5 or 1.7 — 

 to hold the diatoms in place : they slide about in all directions, 

 and a photograph of them cannot well be taken. And even 

 when this medium is viscous enough to hold the frustules, the 

 heat of the lamp soon causes them to move. Will Mr. Dudley 

 state his views on this subject ?" 



Mr. Dudley : " My experience in photographing diatoms 

 mounted in the new medium, accords with the statement given 

 by the President ; and I have found, in addition, that the slight- 

 est particles of dirt, present in the medium, gradually move 

 toward the larger diatoms, and thus mar the picture." 



OBSERVATIONS ON RESOLUTION OF AMPHIPLEURA PELLUCIDA. 



Mr. Wales : " When, about twenty-five years ago, diatoms 

 began to be studied as test-objects, the lenses, which were then 

 of very low aperture, disclosed in Pleurosigma angulatum only 

 one system of lines. Afterwards, lenses of higher angular aper- 

 ture revealed, besides, the intersecting lines ; and, still later, 

 further increase of resolving power made known the beads, or 

 bosses. This, therefore, is the order : first, the systems of lines; 

 afterwards, the bosses. This law applies to all diatoms that have 

 been fully resolved. It must hold true, as well, of Aniphipleiira 

 pellucida. If, therefore, a lens is incapable of showing the two 

 systems of lines in this diatom. I feel sure that it will not show 

 the beads ; and I am constrained to think that those persons 

 who believe themselves to have seen the beads, with such a lens, 

 have confounded illusive images with real ones. I have myself^ 

 as yet, seen, by transmitted light, only one set of the lines. The 

 new medium of Prof. Smith will, perhaps, enable us to resolve 

 both sets. The photograph made by Dr. Van Heurck shows 

 them ; but that was taken with a vertical illuminator." 



