Absorption Spectra. 237 



mined. To take an example, a wedge-shaped band is 

 roughly centred so that the position of the vertical line lies 

 somewhere about its centre. In observing the position of 

 the observing telescope on the spectroscopic scale, it is seen 

 to be 180 . The convergence of the sides of the band are 

 next indicated with the moveable webs, and found to meet 

 at a point two divisions of the transverse line above (i.e., 

 towards the violet) the central vertical line. Each division 

 of the line is found to equal 30' of arc. This must, therefore, 

 be added to 180 , giving 181 as the correct reading of the 

 primitive line on the spectroscopic scale. This can be 

 reduced to wave-lengths by means of an interpolation curve 

 or formula. Such is the method to be adopted with large 

 instruments, and when great accuracy is needful. Where 

 the spectroscopic scale is reflected from the surface of the 

 nearest prism, and such scale is a wave-length scale, which 

 is to be recommended for ordinary purposes, the method 

 becomes much more expeditious, since in such instances 

 the point of convergence of the sides of the bands is directly 

 indicated on the scale. Even without wires, or webs, a 

 rough idea may be formed, at a glance, of the point on the 

 scale to which the sides converge. Such method is often 

 useful in the case of the micro-spectroscope. 



Wedge-shaped Cells. An exceedingly expeditious method 

 of obtaining a wedge-shaped portion of absorption fluid is 

 by simply placing a minute drop of the fluid to be ex- 

 amined on an ordinary microscopic slide, and placing on 

 this drop a plano-convex lens of short radius of curvature. 

 If the front lens (A) of an ordinary achromatic condenser, 

 Fig. 1, Plate VIII., be removed, a drop of the fluid, say 

 blood, (b) may be placed on the upper surface of the sub- 

 jacent lens (B), and, replacing the upper lens, it may be 

 screwed down so as nearly to touch the surface of the one 

 beneath, and thus a plano-concave film of the absorption 

 fluid is formed, any radial section of which constitutes a 



