Universal Aperfomefer. 199 



with a small hole is put on for purposes of centering, and 

 when it is used after the manner of Abbe's apertometer; e is 

 an arc (a good protractor answers very well) graduated to 

 degrees (any higher refinement 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 estimated). An arm 

 moving freely on a pin at the center 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 ordinar)"- glass slide (3 by i), 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 center of the arc, and therefore of the pin on which the 

 movable arm turns. The glass slide is held by a separate 

 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 b, 

 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 my 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, /. 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 onh' available when the front lens is flush 

 with the surface. 



The mode in which I prefer 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 center 

 of the arc, by slipping 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 center of 

 the arc, or pin, on which the arm carrj'ing / moves; 

 their intersection is thus placed directly over the center of 



* An objective which, when measured in the old way by the sector will show i8o° as the air 

 angle, will, measured the same way when half its front surface is covered, give 115'' or so, i. e, 230' 

 for the whole angle ; for the explanation of this see Mr. Wenham's article loc. cit. The true 

 angle is obtained thus, 180° — 115° = 65", and twice this or 130' is the true air angle. 



