Further Remarks on Immersion Apertures. 121 



expressed in his usually clear style ; I will therefore state what I 

 suppose to be asked before I reply. I understand him to call 

 attention to the familiar fact that no ray coming from behind, 

 which arrives at the front surface of an objective at an angle of 

 incidence greater than the angle of total reflexion from glass to 

 air, can emerge into air in front of the objective ; and then to ask 

 me whether I have "observed any such Hmit when dry to be 

 exceeded when the front is immersed ? " To which I reply that in 

 every one of the objectives I have examined, which has a greater 

 balsam angle than 82°, this is the case of course. Surely Mr. 

 Wenham must have known when he asked the question, that even 

 in immersion objectives " with an additional front," with which he 

 concedes a greater angle than 82° can be obtained " in the body of 

 the front," that angle will still be rigidly limited to 82°, if air be 

 substituted for water in front of the objective. 



I repeat what I have asserted from the very first, that I give my 

 unqualified assent to the proposition that 82°, or double the angle 

 of total reflexion from crown glass to air, cannot be exceeded " in 

 the body of the front of a dry lens " ; but I also point out that 

 precisely the same optical laws which fix this limit for the dry 

 front, would fix 122°, or double the angle of total reflexion from 

 crown glass to water, as the limit for the immersion front. That 

 the opticians have not yet succeeded in constructing immersion 

 objectives of this extreme angle is not because such a pencil cannot 

 be transmitted by the front, but because they have not yet learned 

 to correct its aberrations. That up to 100° the aberrations may be 

 corrected successfully, is shown by the objectives of Mr. Tolles, 

 which I have described. 



Mr. Wenham further asks of me — " Can he show us the passage 

 of the rays through one of the object-glasses, such as he advocates, 

 in a diagram of correctly enlarged dimensions ? "—referring, I 

 suppose, to objectives of three systems only, since the case of the 

 others is conceded. I reply that, if Mr. Tolles thinks proper to 

 deviate from the ordinary practice of those who make objectives for 

 sale, and to communicate for publication the details of the con- 

 struction of either or both the objectives described in my November 

 paper, it will be an easy matter for me to gratify Mr. Wenham, 

 and I will endeavour to do so. Not that I think this additional 

 testimony needed to show the accuracy of my measurements of the 

 immersed apertures of these objectives, but because, besides the value 

 of the information to objective makers, I should be happy to be the 

 means of adding to the scanty store of facts which the objective 

 makers have placed at the disposal of science. Mr. Wenham may 

 be justly proud of what he has done in this direction, but he must 

 not be surprised if those who make their living by constructing 

 objectives are less liberal than an amateur can well afford to be. 



K 2 



