228 The Microscope. 



have been overlooked by most, if not all, writers on the subject. 

 Upon examination, however, it would appear to be of consid- 

 erable importance, and as such I venture to direct to it the at- 

 tention of the society. 



THE ZEISS 1-18 OBJECTIVE. 



PROP. ALLEN Y. MOORE. 



HAVING recently had an opportunity of making a careful com- 

 parison of a Zeiss 1-18 and Spencer 1-10, I should like to 

 detail the results for the benefit of intending purchasers who 

 have become dominated by the idea that foreign objectives are 

 far superior to those of our own makers. 



The Zeiss objective is one of those constructed upon the 

 principle of homogeneous immersion, yet not adjustable. The 

 aperture claimed for it is 111°, but in Chance's hard crown glass it 

 only yields 1 10°. The working distance is extremely short, being 

 only about 0.0055 inches — thus preventing its use upon many 

 of my most valued tests. The magnifying power is 200 di- 

 ameters measured at 10 inches from the front lens; thus show- 

 ing it to be really more than the nominal focus would indicate. 

 The mounting is rather peculiar, being only about 1.6 inches in 

 length while its greatest diameter is 1.1 inches. This shortness 

 and its general make-up make it resemble very closely the 

 adapters so frequently seen with the cheap French triplets, 

 although it shows superb workmanship. 



Such a test as an ordinary balsam mounted Amphipleura 

 pellucida is easy for either objective, and as the corrections 

 were somewhat similar, I knew I should have to select a very 

 difficult test to bring out any great difference in the performance 

 of them. For this purpose I selected a valve of Amphipleura 

 pellucida of more than ordinary fineness, balsam mounted, and 

 used the light direct from the mirror — having no hemispherical 

 lens or other illuminator between the mirror and the object. 

 Under these circumstances the Spencer lens gave a good reso- 

 lution of the lines while the Zeiss would not show them at all. 

 Lest it be urged that the 1-10 had the advantage in being ad- 

 justable, I will state that the 1-18 was carefully corrected for 

 thickness of cover, etc., by the draw tube, and the 1-10 was sub- 



