OBJECTS FOE LENS-TESTING APERTOMETER 389 



a dark ground. Nothing is so sensitive. For the lowest powers 

 one of the smaller and more delicate of the Polycistince, because it 

 takes light well, is good. For medium powers a coarse diatom, a 

 Triceratium fimbriatum, is excellent ; for unless an objective is well 

 corrected the image will be fringed and surrounded with scattered 

 light, and the aberration produced by the cover-glass is plainly 

 manifest, and by accurate correction can be done aw^ay. 



Ei*ror of centring is one of the special defects of objectives 

 which the Abbe method of testing does not cover. But if we place 

 a sensitive object in a certain direction, and when the best adjust- 

 ments have given the best image, rotate that object through an angle 

 of 90, only a well-centred objective will give an unaltered image 

 throughout. If not well centrefl it will at certain parts grow 

 fainter or sharper. The most useful image for this purpose with 

 medium powers is a hair of Polyxenus lagurus mounted in balsam 

 (frontispiece, fig. 6). 



For higher powers nothing surpasses a podura scale. In this 

 particular it has always been of great value to opticians. It should 

 be strongly marked, and must be in optical contact with the cover - 

 glass ; this may be tested by means of an oil-immersion and the 

 ' vertical illuminator.' 



The objectives of widest aperture are now more easily tested, 

 because homogeneous condensers with much wider aplanatic areas 

 are now, as we have seen, made by the leading English and 

 Continental opticians ; and there is little doubt but that there is a 

 considerable future before homogeneous condensers. The best that 

 can be done is to take a diatom, such as a Coscinodiscus, in balsam 

 with strong 'secondaries' (Plate I. figs. 3 and 4), with the largest 

 aplanatic cone that can be obtained, which at present can be best 

 accomplished with a semi-apochromatic oil-immersion condenser of 

 1*3 IS". A. It must be a good objective indeed that does not show 

 signs of breaking down under this strain. An illuminating cone 

 of N.A. I'O is probably just below T the point of overstrain with the 

 best lenses at present at our disposal. 



Testing lenses therefore resolves itself into the following methods, 

 viz. : 



1 . For low and medium powers : dark ground with a Polycistina 

 or a diatom, according to the po\ver. 



2. Centring for medium powers (an ordeal not needful for very 

 low powers) should be by means of a hair of Polyxenus lagurus, em- 

 ploying a J illuminating cone. 



3. Centring for high powers : by means of podura scale. 



4. Definition : with wide-angled oil immersions, Coscinodiscus 

 aster 'omphalus with wide-angled cone obtaining sharp, brilliant, 

 and clear view of 'secondaries/ or coarse specimen of Navicula 

 rhomboides, which may be mounted in a dense medium. In 

 testing a lens it does not so much matter what the object is, because 

 the real test lies in the ability of the lens to stand a large direct 

 axial cone. A lens of very great excellence will stand a Jths cone, 

 an excellent lens a Jths cone, an indifferent lens only a \ cone, while 

 a bad lens will not even admit the use of that. A dark ground is a 



