243 



PRESIDENT'S ADDRESS. 



THE PHOTOGRAPHY OF VERY TRANSLUCENT 

 DIATOMS AT HIGH MAGNIFICATIONS. 



{Delivered February 21sf, 1908.) 



The photography of very translucent diatoms at high magnifica- 

 tions is often quite a troublesome matter. In the first place, it is 

 not easy to obtain sufficient contrast between the object and the 

 background ; and, in the second, it is often very difficult to show 

 the secondary markings distinctly and crisply, especially when 

 they require considerable magnification, as is almost always the 

 case on account of their being so exceedingly minute. 



The first of these difficulties is directly proportional to the 

 nearness of the index of refraction of the mounting medium to 

 that of the silex of the diatom. Taking the refractive index of 

 the latter to be 143, and that of Canada balsam as 1'52, the 

 "Index of Visibility" — as the difference has been somewhat 

 dubiously called — is only 9, which is very small ; hence troubles 

 are at their maximum. With styrax by comparison, the differ- 

 ence of the indices produces some improvement in the possi- 

 bility of discovering details, whilst contrast effects are more 

 pronounced. Substituting monobromide of naphthalin (1*658), 

 the differentiating power is again increased, whilst by employing 

 realgar the microscopist has a still greater accession of possi- 

 bilities. It is evident, then, that this arsenical compound is by 

 far the best medium in which to mount diatoms, and it is a 

 subject of regret that more diatom-mounters do not take advan- 

 tage of its high index of refraction notwithstanding the many 

 difficulties attending its use. The photomicrographer, however, 

 should always recollect, if he has a special diatom to photograph, 

 to do his best to obtain it mounted in this medium, as it is 

 far easier to get good results than if it is mounted in any other. 

 Unfortunately, however, it frequently happens that one is unable 

 to obtain a realgar mount of some rare species, or to remount 

 a particularly fine specimen which is already put up in some 



