192 On Zeiss' ^\th Immersion. By W. J. HicMe. 



break down under deep eye-pieces, which popular theory says they 

 ought not to do, and are further characterized by having almost 

 no working distance, while their real magnification is always vastly 

 below the nominal amount. 



I propose to give now a tabular statement of the estimated 

 performance of the three lenses I have mentioned ; namely, 

 Zeiss' ^V inch (C), Seibert's ^V inch, and Schieck's 4V inch. My 

 manner of procedure has been as follows: I have taken 1000 as 

 the highest number of marks that could be assigned to any objec- 

 tive for its performance, and have made my estimate in each case 

 with this number as the maximum. In some instances I have 

 given the average of three different trials. 



Stauroneis spicula (horizontal). 



Objictivks Compared. Marks. 



Zei.ss%'5 inch fldO 



Sfibeit's 21: inch 730 



Schieck's Jq incli G50 



Stauroneis spicula (vertical). 



Zeiss' J5 inch 600 



Seibirt's ^V inch 



Schieck's Jjj inch 



Frustidia Saxonica (transver.se lines). 



Zeiss' 2V inch 970 



Seibert's 2^4 inch 450 



Schieck's -^ incli 800 



Frustulia Saxonica (longitndinal lines). 



Zeiss' V5 inch 800 



Seibert's 2V inch 200 



Schieck's J„ inch 430 



Navicula crassincrvis A (transverse lines). 



Zeiss' Jj inch 950 



SeiberPs J, inch 750 



Schieck's^JL inch 700 



Navicula crassiiiertis B (transverse lines). 



Zeiss'^ inch 960 



Seibert's 2V inch 600 



Schieck's J^ inch 520 



Two of these were examined, the second somewhat finer than the first. 



Amphipleura pcllucida (transverse lines). 



Zeiss' J5 inch 600 



Seibert's -J^ inch 540 



Schieck's J^ inch 500 



Lamplight was employed on each occasion. 



One word here about testing objectives. I have repeatedly 

 heard it asserted, that the only proper course to pursue is to try 

 tbem all, whatever their number be, under conditions exactly the 

 same ; that is, on the same object and with precisely the same 



