THE PROJECTION MICROSCOPE 195 



rate price is also Keichert's oil T V (really y^) of N A 1/25, price 

 1. 5s. only. In every optical quality a selected specimen has 

 been pronounced on the highest authority to be scarcely, if 

 at all, below Zeiss's apochromatics ; and for lantern use it is 

 far better, owing to its longer working distance and flatter 

 field. It is only, and not greatly, excelled on the screen by 

 Powell and Lealand's apochromatics. Leitz's y 1 ^- oil lens is 

 also an excellent projection objective of long working distance ; 

 both these have much more of the latter than Zeiss's y 1 ^- of 

 same N A. This quality is a great point in lantern demonstra- 

 tion. Of water-immersions, the best I have tried are Seibert's 

 ( T y and Zeiss's Gr (|-), but I believe the last is not now made 

 more's the pity, for it was the best of his water series for 

 this class of work. 1 



So far as the lantern and projection are concerned, it is 

 a subject for regret that opticians seem now to have confined 

 their immersion lenses of moderate price to y 1 ^ focus ; only the 

 expensive apochromatics being now made of ^ inch or 3 mm. 

 focus. The latter would be a much more generally useful 

 lantern focus, covering a larger field and giving more light. 

 An old Zeiss oil-lens of J focus, though far surpassed by more 

 modern productions in other respects, gave me a field and 

 brilliancy which no T V can possibly equal ; and I venture to 

 express the hope that increase in projection work may lead to 

 the longer focus being made at a moderate price. Herr 

 Eeichert was good enough to send me for trial a 4 mm. oil- 

 immersion made to a special order, with large glasses. The 

 light passed by it was enormous, bringing many objects other- 

 wise impracticable within the O.H. light. This lens could be 

 supplied to order, where the most must be made of the lime- 

 light alone. But 3 mm. would be more generally useful. 



1 I have no doubt there are many excellent lenses not here named, and 

 take the opportunity of saying that I shall gladly at any time test any which 

 the maker thinks likely, in definition, working distance, and flatness of field, 

 to be suitable for projection purposes. I am only here describing what have 

 actually gone through my hands. 



o2 



