6 Laboratory Spectroscope. 
The graduations of the arc are unavoidably close and difficult 
to read in a feeble light ; consequently the eyes of the observer 
become speedily tired and unfitted for the examination of faint 
spectra. Nevertheless measurements made with the graduated 
arc and vernier are, In my experience, more trustworthy and 
satisfactory than those obtained with even the best photographed 
scale that I have had the opportunity of working with. Desiring, 
then, to retain the method of direct angular measurement I sought 
to multiply the motion in such a manner as to obtain wide readings 
on a convenient scale. After many trials in different directions 
the form of apparatus which I shall now describe was finally 
adopted. | 
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 8,* 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 7; 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 instrument. 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 with the teeth of the 
wheel on the rod 7, and to act upon the teeth directly so as 
to cause the rod carrying the index, A, to rotate easily. This 
strip is bent to a curve whose radius is equal to the distance from 
the axis of the pillar, P, of the instrument to the middle of the 
toothed wheel. The strip is attached to a stout arm and this is 
in turn screwed to the slightly projecting end of the heavy plate, 
D, which carries, and of course moves with, the observing 
telescope, the motion being communicated to the latter by turning 
* 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. 
