292 SUPPLEMENT TO THE 



which the body is to revolve horizontally to the other side 

 of the candle till the opposite half of the field is illuminated 

 as before, and the object-glass has arrived at the position 

 denoted by the dotted lines B, and the axis of the body 

 into the line C G. The space travelled over by the body 

 from F to G is the angle of aperture ; in the present case 

 equal to 60, for it is evident by construction that the 

 angles D C E and E C H are equal to the angle F C G 

 made equal to 60. 



DESCRIPTION OF AN INSTRUMENT FOR MEASURING 

 ANGLES OF APERTURE. 



Figure 20 represents an elevation of an instrument for 

 angles of aperture ;* E is the body ; B the object-glass to 

 be measured ; D an adjustment for the focus ; C the focal 

 point; H a tempered steel needle inserted into a hole in a 

 piece of brass H, which can be adjusted into the line of 

 the axis K, by means of the two screws at I ; F is a plate 

 of brass revolving over another G, on which is a graduated 

 arc, the body being secured to the upper plate by two firm 

 supports, and the inferior plate having also two rests ; A is 

 the flame of a candle, which must be placed truly in the 

 axis of the body, or the field will not be bisected in a 

 vertical line, or the angle of aperture truly obtained. If 

 the object-glass is used without the eye-piece, the flame of 

 the candle appears as a lucid point at one end of the 

 horizontal diameter of the aperture of the object-glass, 

 when in such a position that its flame would bisect the field 

 of view if the eye-piece was attached. 



L is a disc of glass so adjusted, that when it is inserted 

 into the hole occupied by the needle at H, the surface next 



* This valuable instrument maybe made in wood for a few shillings. A. P. 



