286 NOTES AND MEMOEANDA. 



astounding statement as the following : — ' The lens in this instance 

 was properly corrected as a dry lens, and then after measurement in 

 air it was measured in water and then in very fluid Canada balsam 

 without alteration of the adjustment. It may be quite possible 

 that if the lens had been readjusted so as to give the best image for 

 immersion in balsam, a slightly greater angle might have been ob- 

 tained ; but this would not have been a fair way of making a com- 

 parison, as it is not the mode in which the glass would ever be employed 

 in actual practice.' By not saying squarely, It is probahlij true that 

 if the lens had been readjusted so as to give the best image for immersion 

 in water, a greater angle tvould have been obtained ; and this loould have 

 been the fair way of making the measurement, as it is the mode in ivhich 

 the glass would he employed in actual practice, Mr. Brooke lost a rare 

 opportunity to do a noble if not a generous act. As he is well known 

 to be incapable of an intentional soi)histry which by adroitly worded 

 phrase should suggest a doubt where none is felt, belittle the conces- 

 sions which are called for by manifest truth, and say one thing which 

 is true but has no relation to the case at issue, and at the same time 

 imply another thing which does not relate to the case but is un- 

 qualifiedly incorrect, there is no choice but to conclude that his extra- 

 ordinary statement, notwithstanding its tone of judicial coolness, was 

 made without that deliberation which the official character of the 

 Address demanded. 



" On-the other hand, a still more recent lens by the same maker, 

 claiming still more excessive aperture, has been examined by Mr. Wen- 

 ham by his method of cutting ofi" false light. By this method, which 

 would seem incapable of excluding any image-forming rays, he suc- 

 ceeded in obtaining a clear and distinctly limited angle for the lens 

 whose light, when not thus protected, was vague and uncertain ; the 

 angular aperture at the same time being reduced from ' 180° ' to 

 ' 112^,' which corresponded within a few degrees with the aperture 

 computed trigonometrically fi'om the width of the front lens and the 

 length of the working focus. To this it is answered, that with a dry 

 object on the cover there is no distance involved and the triangle is 

 impracticable : while accurate focussing upon a stop which is feasible 

 at ' uncovered ' adjustment, is liable to error from spherical aberration 

 when adjusted for maximum angle. Mr. Tolles' method of demon- 

 strating the utilization of extra-limital rays is by placing a central 

 stop upon the posterior surface of the back system of lenses, so large 

 as to cut oif all light when the objective is used dry; so that by no 

 trick of illumination can the light be made to pass through the narrow 

 ring of clear ajDerture remaining around the stop ; but if water be 

 flowed in both above and below the balsam-mounted object, convert- 

 ing both the objective and the illuminating semi-cylinder into immer- 

 sion arrangements, a well lighted and defined image is immediately 

 produced. With regard to extreme angles in connection ^dth dry 

 objects, Mr. Tolles claims that his much-disjiuted ^ inch does actually 

 form an image with the most oblique rays that can imjunge upon the 

 slide, all other rays being cut oft' by a card or shutter which can be 

 moved up close to the bottom of the slide." 



