Measuring- Apparatus for a Laboratory-Spectroscope. 107 



Description of the Apparatus. — The annexed woodcut, which 

 is taken from a photograph, represents the whole apparatus. 



The index A, attached to the spectroscope, moves in front of a 

 graduated plate of opal glass, the latter being supported, in the 

 manner shown, by the stand S*, to which the spectroscope is 

 also screwed by means of the rod R. The index is attached to 

 a milled head which moves stiffly on a stout steel rod r ; the 

 latter can revolve in little bearings supported by the projecting 

 arm of " angle-brass," a, the other end of the rod being let 

 into a hole drilled in the head of the pillar, P, of the instru- 

 ment. On the rod just mentioned, and immediately beneath c, 

 a small toothed wheel is securely keyed. The diameter of this 

 wheel is about one centimetre, and the teeth upon it are fine 

 and well cut. c is a stout metallic strip, five centimetres long, 

 whose lower edge is serrated so as to correspond accurately 



* The stand is of stout walnut-wood. A rebate of the thickness of the 

 glass plate is cut to the depth of three centimetres from the vertical piece 

 of the stand. The straight edge of the plate is laid in the groove and is 

 there secured, in part by a pin passing from behind through a hole drilled 

 in the glass, and in part by a wooden slip screwed on in front. 



