270 Remarks on the Aperture of Object-glasses. 



with the face of the lens. When now the solar light was thrown 

 through the lens as before, a superb amber-coloured triangle of 

 light started into view, the sharp, well-defined edges of which per- 

 mitted the angle at the focus to be measured with ease by a card- 

 board protractor held beneath the flat tank, or by any similar device, 

 taking care, of course, that the eye should be perpendicular to the 

 edge of the light-triangles at each reading, to avoid displacement by 

 the refraction of the upper glass of the tank, which would have made 

 a small error. The cut, Plate XIX. lower portion, Fig. 1, which 

 is a diagram of the objective and tank as seen from above, will, I 

 hope, make the arrangement clear. The plan has the advantage 

 that no part of the objective is exposed to the balsam except its 

 face (which is easily cleaned by a little coal oil), besides which the 

 measurements are much more quickly effected than with the sector, 

 and are not liable to the errors which affect its use when the lenses 

 are closed. 



By this method, then, I measured the balsam angle of the 

 ^th Mr. Tolles had sent me, with the following results : uncovered 

 75°, or nearly what the sector gave ; completely closed nearly 80 3 . 

 I subsequently extended the measurements to the immersion T jth 

 and T Vth by Mr. Tolles, belonging to the Museum, and found that 

 the maximum balsam angle of each was less than 80°. These 

 results, it will be seen, fell within the limits laid down as possible 

 by Mr. Wenham. 



To measure the water angle of Mr. Tolles's -^th, I now con- 

 structed a thin water tank by cementing strips of glass between the 

 edges of two sheets of plate glass about three inches square, so that 

 they should be held about the sixth of an inch apart. All four 

 sides were closed, but one side had in the centre an opening half an 

 inch long, and the edges of the stripes adjoining this were bevelled 

 as in the cut, Fig. 2. When this tank was filled with water, I had of 

 course a thin sheet of water, which would not run out when the 

 tank was held horizontally, and by levelling this, as had been done 

 with the balsam tank, in front of the objective, the angle was measured 

 in the same way. The luminous pencil was by no means so 

 brilliant as in the case of balsam, but its limits were sharp and 

 clear, and it could readily be measured. With the T Vth the results 

 were about 90° at the uncovered point, nearly 100° when the 

 objective was corrected for the thickest cover through which it 

 would work. Neither the ^th nor the T Vth exceeded 96° when 

 closed as far as possible. 



I promptly communicated these results to Mr. Tolles, and was 

 immediately requested by him to examine yet another objective, a 

 £th, which reached me March 22nd. 



On measuring this objective in balsam, precisely as I had done the 

 others, I got somewhat over 90° at the uncovered point, somewhat 



