On Zeiss isth Immersion. Btj W. J. Hickie. 193 



kind of illumination ; and at first sight such a course appears to 

 recommend itself to the judgment of all. But it will require no 

 great amount of consideration to convince any reasonable being 

 that no method could be more unfair or more productive of false 

 results. Every high power has its own peculiar idiosyncrasy, has 

 its own particular way in which alone it can be induced to do its 

 best ; and the illumination which is admirably suited for one glass 

 may be very unsuitable for another. Or are we to expect all 

 objectives to do equally well under all possible conditions of iUurai- 

 nation ? for that is pretty much what it amounts to, seeing that 

 almost every microscopist has his own pet mode of illumination. 

 To take an instance from the objectives in my own possession : 

 Schieck's (so-called) 4V inch immersion works best Avith an 

 Abraham's prism, brought up as close as possible to the stage, the 

 top edge of the prism being half an inch higher than the object, 

 and set at a particular angle. Zeiss' aVth (C) is at its best when 

 the angle formed by a line drawn from the object and one drawn 

 along the bar of the mirror is exactly two degrees less than a right 

 angle ; and it shows itself very sensitive to any departure from 

 this precise arrangement. Seibert's 2?th, again, exhibits its best 

 performance when the above-mentioned angle is some three degrees 

 greater than a right angle. My plan therefore has been to find 

 out under what conditions each lens performs best, and to let each 

 be tried in its own way, and then to estimate the results : a 

 troublesome plan, no doubt, but, as 1 think, the only fair one. 



I may now briefly state what I have been able to do with this 

 C lens. 



(1) Kesolved Holler's (so-called) Nitzsclia curvula into dots, 

 and that too with the greatest prominence. 



(2) Eesolved every three out of five frustules of Am^liipleura 

 pellucida. 



(3) Eesolved the transverse lines of Staiironeis spicula, with 

 the frustule lying vertical ; that is, with the lines in the same 

 direction as the illumination. 



The first, I dare say, does not amount to much ; and the 

 second, perhaps, is not more than people usually expect from a 

 first-class objective ; but that there are many glasses able to do 

 the third, and with the same means, is what 1 shall believe — when 

 I see them do it. 



I wish also to have it distinctly understood, that by " resolved " 

 I do not mean " a wheen o' skarts " dimly visible through a dirty 

 mist, but a complete resolution. 



In doing the second I employed a silver mirror, with bright 

 sunlight modified by blue glass. For the first and third I had 

 merely a Beckett lamp, with a silver mirror assisted by certain 

 paper shutters. 



