250 T. F. SMITH OX ARACHNOIDISCUS. 



and set at its best point ; that an oil-immersion is as sensitive on 

 it as a dry objective is on the Podura scale, and that the best point 

 once found and noted in this diatom, mounted in balsam, will be 

 the best point within very narrow limits on all other objects 

 mounted in that medium, or in any other with nearly the same 

 refractive index. 



Of course, I am pre-supposing central light, and a good wide- 

 angled, achromatic condenser, without which conditions no wide- 

 angled objective will work perfectly. 



I do not say that every disc of this diatom will act as a test, any 

 more than will every scale of Podura. Some will show no pro- 

 jecting species even with the widest-angled objective, and others 

 are. so coarse as to be no test at all ; but a properly- selected one 

 will answer all the purpose, both for denning and resolving power. 



The one used by myself is about the T ^- -" in diameter (the one 

 under the y is only t\q" in diameter), is divided into 26 bays, 

 and has about 7,000 perforations, the smallest about ^oWo" m 

 diameter, none of which has less than four points projecting 

 inwards from the sides, and some five or six. 



"When I first thought of bringing forward this test I had no 

 intention but to confine it to oil-immersion objectives, but some dry 

 object glasses have lately come into my hands which show such an 

 advance on what I thought possible, that I have enlarged the 

 scope of this paper to make it include the same object as a test for 

 all high powers. 



That the capacity of a dry objective to show these projections is 

 a great advance in definition is proved by the fact that none of the 

 text books portray them, and it is somewhat remarkable that Mr. 

 Morland in his paper makes no mention of them, although his 

 observations were, I believe, made with a water-immersion. 



This advance is, I believe, due entirely to the new optical glass, 

 of which the objectives are wholly or in part composed, and which 

 has enabled the makers to enlarge the numerical aperture almost to 

 the limits possible with a dry glass, without increasing the aberra- 

 tions. The gain is such that the new dry objectives have entirely 

 overleaped the boundary between themselves and the oil-immer- 

 sions, and have made the old water-immersion lenses obsolete. 



I do not speak this without warrant, having carefully compared 

 them with the last formula water-immersion \" of Powell and Lea- 

 land, and with Gundlach's water-immersion T y, both glasses 



