The Angular Aperture of Objectives. 59 



between tlie common and the " peculiar " kinds, i. e. ref. index 

 of the " balsam " used being the same as " glass," — the one of 

 usual construction would show less (probably) than 82°, while the 

 one I alluded to in the note which Mr. Wenham comments upon 

 would give over 100°, — I got 110°. 



The latter being transferred to the microscope (no change of 

 adjustment) would practically have no more than closely the same 

 angle as the first, for with both the outside plane surface of the 

 slide limits the balsam pencil to angular cone resultant from an air 

 angle of 180^, and which cone is less than 82^ in the glass and in 

 the balsam. 



All this is " piper's news," of course, but it seems a story 

 necessary to be told again. But to proceed beyond this — with the 

 semi-cylindrical, or some equivalent " thing," we have that angle 

 ultra of anything derivable from 180° in air, and which is utilized 

 by this objective (the one reported of in my last, ' Monthly Micro- 

 scopical Journal' for June, current), and not touched with the 

 other sort. 



What I utter is a matter of knowledge, not speculation. I 

 have seen well with that additional 20° of pencil, though as yet by 

 chance not verified by others' eyes. All the 80° and more being 

 shut out, the outside portion illuminated the object, and the 

 objective with that ultra portion of the cone (and limited to it) 

 gave a good image of the object. But the cylindrical appliance 

 was necessary to show that the part heyond 80° was used. 



One word more, to show there is no room to question. When 

 used as a means of measure of balsam angle of the objective, the 

 light being put down the microscope hody, the cylindrical surface 

 is dulled to act as a screen on which the angle-limits are marked, 

 and with no deviation, all the while, mind you, the object being in 

 focus. Mr. Wenham's explanation does not apply. It seems to me 

 this is one of those " true facts " he is bound to recognize. 



I do not care to impeach the Wenham tank. The objectives 

 I tried in that tank all showed more angle in " balsam " than was 

 shown by the " wretched adaptation " that vexes so Mr. Wenham. 

 Thus, the I of the tank list (' Monthly Microscopical Journal,' 

 No. XL v., p. 106), the tank showed it to have 110°, while the so- 

 smartly tabooed "cylindric affair" gave it 107° instead! My 

 cause was helped most by the showing of the tank. 



The tank will give perhaps correct exhibit of angle for any 

 liquid, but to test the question at issue here the balsam must have 

 that refractive index involving interior total reflexion at incidence 

 of 40° and a fraction, or, in other words, hke " glass," as repeatedly 

 laid down by Mr. Wenham — not much turpentine, neither must it 

 be a resin. Such is the difficulty with the tank. Your balsam as 

 to index will not agree with case as stated. 



