78 PREPARATION OF DIATOMS. 



However, Prof. H. L. Smith has invented a medium which possesses 

 an exceedingly high index of refraction, viz. : 2-25 to 2-4. We have 

 given a detailed description of the way in which this medium is pre- 

 pared in the last English Edition of our Treatise on the Microscope, 

 and we refer to this work for the details on this subject ('). 



Systematic preparations of Type Slides and selected d'atoms. Every 

 microscopist is aquainted with the admirable type slides which were first 

 introduced by Mr. J. D. Moller of Wedel, in 1867. I have a most vivid 

 recollection of the appearance of the first slides (many of which are 

 in my possession) and the seniation which they produced in the 

 microscopical world. 



Mr. Moller employs a specially constructed microscope to make his 

 type slides, and he adopts his own particular methods, which he has 

 hitherto disclosed to but three or four persons, of whom I am fortunately one. 



I have now known it for a long time but am under an obligation 

 not to reveal anything in connection with the subject. 



Other professional preparers and enthusiastic amateurs have applied 

 themselves in the same direction, and if their results are not as per- 

 fect as Mr. Moller's a fact which anyone who has been initiated into 

 his secret must recognise they at least approach very closely to per- 

 fection ; the type slides of Thum of Leipzig for example are deserving 

 of all praise. 



Type slides proper have not hitherto, so far as we know, been made other 

 than by Moller and two enthusiastic and skilful amateurs, Mr. J. 

 Kinker, of Amsterdam, and Mr. E. Weissflog, of Dresden, both of 

 whom use Moller's process, and have produced genuine works of arts. 



Mr. E. Weissflog, especially, has made himself quite master of the art, 

 and has produced most important type slides, as for instance that of Santa 

 Monica, which contains about 600 forms. Such work requires long and 

 difficult application. 



In connection with type slides we must not omit to mention the prepara- 

 tion of selected diatoms, i.e., preparations containing one or more isolated 

 diatoms. These preparations are very valuable, and every diatomist ought 

 to know how to make them. 



The methods moreover are very simple, and have been fully described 

 by Mr. H. Peragallo, the eminent French diatomist, in his work on 

 " Les Diatom'ees de la Bate de Villefranche" ( 2 ) and I shall now quote 

 all the passages in which our learned correspondent describes them : " I 



( ) The Microscope, London, 1893, p. 305. 

 ( 2 ) Paris, Librairie BaillieYe, 1888, 



