J. P. Cooke on the Spectroscope. 311 
cling the wheel at equal distances from each other and numbered 
from 1 to 
Having described the construction of the instrument it will 
now be easy to understand the method of using it. Let us sup- 
pose that the object is to measure the angle of minimum devias 
tion of the blue ray of the strontium spectrum. By examining 
any chart of the spectra of the chemical elements it will be 
found that this line is situated roughly at somewhat less than — 
two-thirds of the distance from A to H. If, then, we turn the 
angular deviation is diminished, and leave the wheel in the po- 
measured on each prism sepa 
given on page 309,is precisely 
Sa = those —— : seat 
_ Only differs from this by 20”. LS 
ae When it is not important to use absolutely the whole aper- 
ture of the prisms it 1s not n 
ecessary to change the position of 
ing from one part of the spectrum to au- 
2 As the mean of the two lines, spi: 
