776 RECORD OP CURRENT RESEARCHES RELATING TO 



which forms a part of the new instrument, and by means of which 

 one may bisect a minute white circle with the greatest accuracy. 

 Moreover, few would be able to graduate the " apertometer " cor- 

 rectly, and the process of computation, though easy enough, is not 

 necessary. 



A brass tube a (Fig. 7), say 2 inches long, is supported on a 

 pillar, into which another tube h slides easily, carrying at one end 

 the objective c ; the other end h is open except when a cap with a 

 small hole is put on for purposes of centring, and when it is used 

 after the manner of Abbe's apertometer; e is an arc (a good pro- 

 tractor answers very well) graduated to degrees (any higher refine- 

 ment is quite useless, as in the larger angles there will always be an 

 uncertainty of at least a quarter of a degree, a space readily esti- 

 mated). An arm moving freely on a pin at the centre of the arc, 

 carries at its end an eye-lens in a small sliding tube, and having a 

 small eye-hole /, the lens having its focus over the central pin ; g is 

 an ordinary glass slide (3 by 1), which can be slipped in or out of 

 place at will, and is held at right angles to the plane of the graduated 

 arc by two springs, which press it against two uprights, so that the 

 front surface of the glass is exactly over the centre of the arc, and 

 therefore of the pin on which the movable arm turns. The glass 

 slide is held by a sejiarate brass holder, which can be pushed forward 

 when the focal point of the objective is just over the pin until the 

 slide touches the front lens, and a black bar with a straight edge 

 painted on the glass can be made to cut off just half of the surface of 

 the front lens, by putting in the perforated cap at h, and looking 

 through /, which is supposed to be standing over the middle of the 

 arc. This is for using Mr. Wenham's method, and it gives very 

 nearly the same results as his (Prof. Smith's) own. The apparent 

 aperture of the uncovered half is measured (twice this will give an 

 extravagant angle), the whole aperture is then measured, but in the 

 usual way, i. e. until the light disappears ; the angle of the half is 

 now subtracted from that of the whole, and twice the remainder is 

 the true angle; this method is only available when the front lens 

 is flush with the surface. 



The mode in which he prefers to use the instrument, however, 

 and which gives the true air angle, is as follows : — The front surface 

 of the slide g is brought accurately over the centre of the arc by slip- 

 ping the brass holder quite home ; two fine cross lines ruled with a 

 diamond on the glass, are, by sliding the glass laterally, brought 

 directly over the centre of the arc or j^in on which the arm carrying 

 f moves ; their intersection is thus placed directly over the centre of 

 motion ; the objective is focussed on these lines : it is not necessary 

 to use an eye-piece unless the focal length be very long. Yet for 

 true angle, independent of definite length of tube, it would be suffi- 

 cient simply to focus upon the lines without an eye-piece ; the screw 

 j may be used for this purpose, or it may be effected by simply 

 sliding the tube h in a. Supj)ose now the eye-lens /to be over the 

 middle of the arc ; on looking through towards the objective one will 

 see something like Fig. 8, where the outer circle is the periphery of 



