The Optical Quality of Mr. Tolles ^ Objective. 17 



near-contact approach to middle. The same back-and-middle and 

 angular pencil are concerned, and nothing changed except relative 

 position effected by closing the adjustment. The clear opening of 

 the front being the same as before for 90°, its boundary is reached 

 by the pencil indicated by the lines R" E" proceeding from the 

 middle. Projecting the course of these rays their focus is found 

 at F, and shows a reduction of angle from 90° to 72^, or slight 

 fraction more. Although closed not the maximum balsam- 

 angle. 



Spherical Aberration. 



Having shown that the closed point is not necessarily the 

 adjustment for maximum angle, I wish to call attention to the 

 enormous aberration under which angle is taken ordinarily, without 

 " cover," and, I will assume, the systems closed. 



With a dry front surface the same extreme rays R" R", Fig. 2, 

 will have, when proceeding from within to air, refraction to the 

 corresponding air-angle ; but not any ray nearer to the axis than 

 those, can have the same focus in air. 



Take the case of light radiating from a small point in the axis 

 of the microscope at the place of the eye-piece, or, which is the 

 same, the longer conjugate focus. The focal point at the front 

 surface vdll move to a point of gradually increased distance, as the 

 rays from the 10 inches distant radiant point are incident at the 

 convex-surface of the front at a gradually decreased augidar distance 

 from the axis. 



This enormous spherical aberration arises at the front plane 

 surface, and for compensation, or correction rather, needs a covering 

 glass of suitable thickness ; with a technically '' dry " objective this 

 would be sufficient. But with an immersion objective having 

 immersion contact with " cover " to a dry-mount, the dry front 

 surface of the objective is in effect removed to the surface of the 

 covering glass next the object ; and the focus of the extreme mar- 

 ginal rays will then be very near to the focus of the mean and 

 nearly central rays, because all the rays of the cone emerge within 

 a comparatively very small area about the axis. This is all very 

 well known and, though the demonstration is easy, that may be 

 omitted. 



I do not care to comment on Mr. Wenham's precautions in 

 taking angles, as described in his account of the |-inch. I will 

 only say that the whole seems to me quite unnecessary.* What 

 I have said of interposition of " cover " is suggestive of the true 



* Mr. Wenham, in bis diaa;ram at p. 114, the lower one, draws the ray d for 

 170° of aperture at an impossible emergence from the convex surface. It falls 

 outside of the perpendicular and must be bent outward, the surl'ace acting as a 

 concave. Perhaps Mr. Wenham thinks this of no consequence and — so do I ! 



c 2 



