984 PROCEEDINGS OF THE SOCIETY. 



nor can it be greater on the immersion system, wliere an interchange 

 of front adapts it to both conditions." 



In vol. V. J). 118 the following sentence has been referred to : — 

 " I challenge anyone to get through the object- -lass with the immer- 

 sion front a greater angle, or any portion of the extraneous rays that 

 would in the other case be totally reflected, as no object-glass can 

 collect image-forming rays beyond this limit." At the top of this 

 same page I show distinctly that this remark applies to a dry lens, 

 and the whole article bears reference to a former one (vol. v., page 23), 

 with diagrams used for objectives used on dry objects, or such as those 

 generally in use at that time. 



Mr. Tolles having claimed 100° or more of aperture for an im- 

 mersion lens, in vol. vi. (1871) p. 85 I point out the cause arising 

 from an error in the means of measurement. On the next page I 

 give a series of measurements of the angles of objectives immersed in 

 water, all of which wore within the corresponding balsam limit of 

 82 \ Again this referred to the then usual form of objectives that 

 could hi used dry. 



In vol. vii. (1872) p. 272 I say, " Mr. Tolles has accepted the 

 only condition under which the full aperture can be brought to bear 

 on a balsam-mounted object, viz. that of the tiny hemispheres. I am 

 glad of his announcement that he has succeeded in this, and should 

 like to see the same thing done in this country, particularly with 

 large aperture glasses, say higher than Jth." 



In vol. ix. (1873) p. 268 Colonel Woodward having measured 

 the Tolles -^\ referred to in the i:)revious controversy, finds the balsam 

 angle only 76°. On p. 270 he continues, " I subsequently extended 

 the measurements to the immersion J^ and -^\ by Mr. Tolles belong- 

 ing to the Museum, and found that the maximum balsam angle was 

 less than 80°. These results, it will be seen, fell within the limits laid 

 down as j)ossible by Mr. Wenham." 



In vol. ix. (1873) p. 273 there is a memorandum by Professor 

 Keith, in which he says, " Mr. Wenham's experiments alluded to in 

 his article in the 'Monthly ' for January, indicate an explanation, and 

 it seems singular that they did not suggest to him long ago a method 

 of obtaining what Mr. Tolles has obtained — an objective of large 

 angle for objects covered in balsam." Professor Keith's accompanying 

 diagram exactly explains the jjrinciple, and is just the same in effect 

 as that of my additional front lens, described and illustrated in 1855, 

 showing that he had not found leisure to look into former points of 

 the question. My diagram is illustrative of Mr. Tolles' subsequent 

 4-system immersion objectives, the focus being nearly in the centre 

 of the radius of the front lens in both cases. 



In vol. X. (1873) p. 12 I say, "I trust that Colonel Woodward 

 having aflBrmed ' that the position taken by me is certainly true, for 

 objectives as ordinarily constructed,' will allow that this additional 

 lens embodies a deviation from the original question, which was to 

 the effect that there would be no loss of angle aperture of ordinary 

 objectives by the immersion of the front surface in fluids." 



As regards the latter pliase of the discussion, relative to false 



