236 Dr. Alex. Hodgkinson on 



lines into a series of equal parts. The shaded portion E 

 represents the wedge-shaped absorption band, the primitive 

 absorption line of which it is intended to " centre " so as to 

 ascertain its position on the scale of the spectroscope. 

 Having moved the telescope of the instrument so that the 

 vertical line A, B, is approximately in the centre of the 

 absorption band, it is next necessary to determine the point 

 of convergence of the sides or edges of the absorption band. 

 Before doing this, it is advisable to arrange the hollow 

 prism containing the absorbing medium so that its re- 

 fracting edge occupies a position as regards the ends 

 of the slit, say, on a level with the transverse line, as 

 is the case in all the foregoing examples. One portion, 

 F (the lower), of the observed spectrum is that of 

 the medium, the other portion, G (the upper),* the un- 

 obscured spectrum of whatever light source may be 

 employed. For indicating the point of convergence of the 

 band, the eye-piece is provided with two webs, or wires, HB, 

 IB, which are freely moveable in the focal plane, so that they 

 may be arranged along the edges of the absorption band. 

 In the case of faint indefinite bands, lines of as equal 

 degrees of absorption as possible on either side of the band 

 are selected. Since, moreover, towards the apices of the 

 bands the converging sides become curved, it is advisable 

 to take the line of convergence from the base of the wedge- 

 shaped bands, where absorption is more definite and each 

 side apparently a right line. The webs, or wires, being in 

 position along the edges of the band, they converge probably 

 to a point situated to one or other side of the central line 

 A, B. Say this is four divisions of the divided transverse line 

 to the right of the central line. The value of these divisions 

 in terms of the scale of the spectroscope being known, this is 

 allowed for, and so the correct position is readily deter- 



*The terms "upper" and "lower" here used refer to the apparent upper and 

 lower halves, the object appearing reversed by the instrument. 



