PROCEEDINGS OF THE SOCIETY. 



347 



would b3 spoilt. Therefore Herr Zeiss used a very Lard nickel 

 alloy. 



As to aqueous fluids, great caution was needed, as some of them might 

 corrode the metal setting, and unship the front leus ; some, such as 

 zinc chloride, would be very hygroscopic, and, after a few minutes in 

 a dry or moist atmosphere, would vary so rapidly in refractive index 

 as to be useless for such a purpose ; others would penetrate by capillary 

 attraction past the edge of the lens, and gum up the margin of the 

 back surface, so as to reduce the working angle of the lens and intro- 

 duce diffraction phenomena. Therefore an oily fluid would probably 

 after all prove the best. Shellac was proof against cedar oil, and 

 would answer perfectly for mounting objects, and perhaps also for 

 consolidating the front lens in its narrow setting, so as to prevent 

 capillary action at its margin. 



A letter was read from Mr. Adolf Schultze, of Glasgow, well 

 known as an expert manipulator, in which he said that " though he 

 had not had time to examine these fluids closely, yet he was able to 

 say that by their use with the -^^ he at once resolved A. pellucida 

 and other fine diatomaceous tests as distinctly as with cedar-wood oil. 

 The cadmium chloride in glycerine and sulpho-carbolate of zinc in 

 glycerine being very thick and sticky, might, he thought, suit the 

 I well, as it has a very large working distance. Copaiva balsam oil 

 he thought on the whole the best substitute for cedar-wood oil. 

 Although these liquids do not act as solvents on the Canada balsam 

 and the varnish rings of objects, and (with the exception of the 

 copaiva) are free from smell, yet he doubts whether their use ofiors 

 any important advantages over cedar-wood and fennel oil, whose 

 smell is not oflensive if employed in very small quantities. Three of 

 the four fluids require to be washed oft' from the slide and the front 

 leus with water, whilst for copaiva and the other oils a little blotting 

 paper suffices." 



With regard to the ^^ oil-immersion, Mr. Schultze also said : — ■ 

 " It is my opinion that this lens is at present perhaps the finest 

 immersion objective of the same focus made, and that it is not likely 

 soon to be surpassed. My specimen (No. 3) has a magnifying power 

 of 980 diameters, with Ross's A eye-piece, and a working distance of 

 about • 004 inch, its definition is very fine, and its resolving power 

 is as remarkably great as that of the } and the j^ of the same series. 

 Its field is quite flat, as far as I can sec on the tests at my command, 

 and it gives a great deal of light, so much so that when using a 

 microscopic lamp with a wick half an inch broad, the markings of 

 A. pellucida are still visible under Ivoss's F eye-piece, or under a 

 ])ower of 8000 diameters. A2)art from magnification and working 

 distance there seems little to choose regarding other optical qualities 

 between Zeiss's three objectives of i, yV, and -^ inch focus on the 

 homogeneous immersion system." 



Mr. Stephenson, in reading his paper on " The Vertical Illumi- 

 nator and Oil-Immersion Objectives " (see p. 266), said that it was now 

 found that the kind of illumination furnished by the vertical illuminator 



