PROCEEDINGS OF THE SOCIETY. 117 



Perhaps, also, a microscope such as Swift's " Arsenal," with tilting-stage, 

 mii^-ht assist in securinc;; a level field of view. These two suggestions, 

 iiowever, had not been tested by the author, but are given for what they 

 might be worth. 



The President, in inviting discussion on the communication, said 

 this was a subject which the Society had need to have brought before 

 it. Mr. Rawlins had given enough openings, and he hoped some 

 Fellows would discuss it. 



Mr. Blood asked how Mr. Rawlins secured absolute uniformity and 

 eveimess of the layer of collodion or gun-cotton. Also, was it thick 

 enough to give a cover-glass effect, and necessitate the tube-length 

 correction ? Further, did it give complicated reflections between the 

 upper and lower surface ? 



Mr. Rheinberg said that recently he had occasion to measure up some 

 films of collodi>on in other solvents which acted similarly to amyl-acetate. 

 They were stripped off the glass and were found to be about ^trVfftli of 

 an inch. He thought it might be possible to get amyl-acetate or 

 collodion almost as thin as that. 



Dr. Leeson asked whether ordinary thin balsam would not do as 

 well. 



Mr. Scourfield said he had obtained striking effects with the vertical 

 illuminator on living organisms. But there was the difficulty of reflec- 

 tions from the cover-glass, and some internal reflections from the 

 objective, which produced haze and rendered observation of the speci- 

 mens difficult. If that difficulty could be overcome, there would be a 

 great field for biologists working with the vertical illuminator. 



The President said the balsam would take a long time to dry, and 

 there were dust possibilities. Evaporation took place quickly with amyl- 

 acetate, and the film was more or less homogeneous. It was impossible 

 that any amyl-acetate film could dry to anything like the thickness of a 

 cover-glass. A 5 p.c. solution put on of appreciable thickness would 

 dry down to very thin dimensions. He would have liked to hear 

 Mr. Rawlins say something about the relative advantages and disad- 

 vantages of the type of vertical illuminator in which the prism was 

 used, as compared with that employing a cover-glass, or a mica film. 

 He considered that the difference in efifect was, that with the prism 

 illuminator one did not get the illumination truly normal to the surface, 

 and one was only using half -aperture of the objective ; while with the 

 other type there was the possibility of aberrations induced owing to 

 irregularities of the cover-glass, unless it were an optically-worked one. 

 He knew that practical metallurgists varied in their opinion as to the 

 relative. advantages of these two types. The object of covering with 

 celluloid was not clear to him. If it was simply to prevent tfirnishing, 

 the thinnest form would do, but if it was to take the place of a cover- 



