SUPPLEMENT. 391 



On the contrary, objectives of the widest apertures, and capable 

 of yielding intense definition, require the strictest attention to 

 their adjustment;. * 



Nor is this all. It is quite possible to accurately adjust a first- 

 class objective, and, nevertheless, in this very act defeat the 

 maximum performance of a first-class glass I 



When working with oblique light, the maximum performance 

 of a superior glass will be obtained at maximum aperture ; if 

 this maximum aperture obtains, with the systems ' closed," as 

 is often the case, it is then manifestly our business to use covers 

 of such thickness as will " correct" the objective at tk closed," 

 and to work such a glass over thinner covers, requiring the lens 

 to " correct " at, or near * l open point," would surely defeat its 

 best performance. 



It being possible that we may have time in the course of the 

 evening to practically discuss this point, I have brought here an 

 objective which, in any position of its collar, has plus 180 of 

 aperture ; its balsam angles are at " closed," 97i; at half way 

 between " open " and ;i closed " 95 ; at open point, 85. It will 

 be an easy matter to demonstrate that this glass is defeated by 

 the thin covers of the Moller plates, over which the glass will 

 " correct " near to " open point," and conversely, of the decided 

 increase of definition obtained (and angle also) when worked 

 through a supplemental cover of sufficient thickness to cause 

 adjustment with the systems at, or near " closed." 



I have brought here another, and a totally different objective; 



this glass has a constant bals.im angle of 100 through nearly 



the whole run of its adjusting screw. Manifestly, when using 



this glass, there need be less attention paid to the thickness of 



"cover, a fact which is demonstrated in practice. 



* In an article contributed by Mr. P. EL Wenhara ?ee London Micro.scop- 

 ical Journal for March, 18761 read, as follows : "The adjustment seems to 

 be a stumbling block for those advocating- an extra immersion theory. We 

 have now in use thousands of serviceabla tm.nerdion object-glasses, capa- 

 ble of denning most tests, and whi'.n have no adjustment, as they are set 

 for an average thickness of cover. They answer well, because in the im 

 mersion system the errors of cover aberrations aro nearly eliminated, and 

 with a balsam intermedium they would be inappreciable." (II!) 



J. E. S. 



