8 Laboratory Spectroscope. 
with the aid of the arc. For this purpose the telescope is moved 
into such a position that the rays less refrangible than the red 
Potassium line shall occupy the field of view; the zero of the 
vernier is then made to coincide with the nearest convenient 
degree marked on the are. The rod 7 is then firmly grasped and 
the index A brought down to a horizontal position, and a fine 
dot made on the plate under the point by means of a pen dipped 
in “black japan.” This point is taken as the zero of the scale. 
Each half degree is marked off in a similar manner until the 
semicircle is graduated. The two scales are again compared at 
different points, and the opal glass plate removed; each large 
division, corresponding to half a degree, is then subdivided into 
10* equal parts. Finally, the semicircle is numbered from zero 
up to 200; each division of the scale therefore corresponds to 3 
of the arc g. In my spectroscope the angular motion of the 
observing telescope is magnified 25 times, and the width of each 
division of the glass scale is 23 millimetres, so that the readings 
are easily made in a feeble light without straining the eyes of 
the observer. 
Reading off positions of spectral lines —In commencing an 
observation it is always desirable to see that the point of the 
index A stands at the zero of the glass scale when the telescope 
is in the corresponding position on its scale. Any adjustment 
of the index that may be necessary is easily made in the way 
already described, namely, by firmly holding the rod 7 and turning 
the milled head which carries the index to the desired extent. 
The actual reading of the position of a line to which the point of 
the fine needle in the eye-piece is brought is then made from 
the glass scale. 
An exceedingly feeble light suffices to enable the operator to read 
the wide divisions on the white scale ; but in observing very faint 
lines I do not read by reflected light, but faintly illuminate the 
scale by means of a very small gas jet or lamp placed behind it. 
Sufficient light is transmitted by the opal glass to enable the 
readings to be easily and quickly made, while the eye of the 
operator is retained in a sensitive condition for feeble rays. More- 
over, in reading it is not necessary to move the head away from 
the eye-piece of the instrument. 
* In the woodcut only five subdivisions are shown. 
