On Immersion Objectives. 215 



mum angle it works through covers rather thicker than the yVth of 

 an inch. This capabihty of working through very thick covers is a 

 great advantage, permitting it to be used on many of the older pre- 

 parations in the museum, with which no ordinary objective of 

 similar power could be employed. I send herewith a photograph * 

 oi Am])hi^leura pellucida, intended to illustrate its defining power at 

 the point of maximum angle. I purposely selected two frustules, 

 which have served me to make many photographs with different 

 objectives, some of which I have sent to London on former occasions. 

 The slide has a cover about o Jo" thick, much too thin for the point 

 of maximum angle with the present objective. I therefore laid on 

 top of it, by water contact, a second cover, t^o-" thick. The 

 photograph was taken through both covers, with all the resulting 

 disadvantages of the additional surfaces ; and the comparatively high 

 magnifying power (1380 diameters very nearly) was obtained by 

 distance only, no eye-piece being used. I beg you to show this 

 photograph to any microscopists interested, and should like to ask 

 those who examine it, whether they ever saw Amphipleura as well 

 defined by a rVth. 



The second objective is an immersion roth, recently sent me for 

 examination by Mr. Tolles. Its balsam angle, by my method, is 

 87° at the open point; over 100^ when the screw collar is fully 

 closed. This also is an objective of three systems only, but the front 

 is a compound one. It works only through very thin covers, at the 

 maximum rather less than sou" thick. I do not think its defining 

 power equal to that of the yVth last described ; nevertheless, at the 

 point of maximum angle it resolves Amjphipleura jpellucida quite as 

 well as most of the roths I have had the fortune to handle. I do 

 not for a moment suppose that this objective is the best that can be 

 made with a balsam angle of 100'' and only two posterior combina- 

 tions. It is, however, good enough to indicate the importance of 

 further work in this direction, while the objective of 87"^ balsam 

 angle appears to have gained superior defining power by its 

 increased angle. 



I have next to say a word with regard to a point in Mr. 

 Wenham's article in the July number, to which I take exception. 

 When he says that in measuring angles of aperture his " custom is 

 (and wiU be) to set the objective in adjustment on a standard test 

 of known average thickness of cover," I am obliged to think him 

 quite in the wrong. However accurately the angle may be 

 measured for the particular position of cover correction selected, the 

 result can give no notion of the maximum angle with which the 

 objective can be used, and such a method is especially inapplicable 

 to those objectives which have a wide range of cover correction. 



* The photographs sent are to be seen at the E. M. Society's Kooms. — 

 Ed. ' M. M. J.' 



VOL. X. S 



