Notes. 



391 



black ground, so that it may easily be read in a dim light. A 

 vernier reading to tenths is carried above the scale by the telescope 

 itself, and immediately beneath the eye-piece is a magnifying lens, 

 which follows the vernier in its movement, and through which the 

 scale and vernier may be read with the greatest ease, without any 

 change of the observer's position. The eye-piece has cross wires of 

 the usual kind. 



It will be seen that the radius of the arc is much greater than 

 that of the table in most table spectroscopes, so that much finer 

 readings are possible ; also, that as the vernier is carried by the 

 telescope itself instead of by a separate arm, there is less chance of 

 inaccuracy. 



Fig. G3. 



The telescope is carried round by means of a screw, which bears 

 -against a lug clipped upon the tube, and works through a small 

 boss cast on one arm of the arc, the opposing spring being attached 

 to the opposite arm and bearing on the opposite side of the lug, an 

 arrangement which altogether relieves the telescope from any lateral 

 strain. The screw has two milled heads ; the smaller is rapidly 

 rotated between the finger and thumb in order to move the 

 telescope quickly through a large distance to another part of the 

 spectrum, whilst the larger is for exact adjustment upon any line it 

 may be desired to read. A blackened screen of sheet metal protects 

 .the eyes from direct light. 



The instrument itself was made by Browning. 



