Absorption Spectra. 233. 



any level from apex to base of the band, and thus allow of 

 the band being accurately measured in its length. Its width 

 was measured in the usual way by the scalewhich indicates the 

 angular position of the observing telescope. A chart was 

 next constructed, having the dispersion scale of the instru- 

 ment reduced to wave lengths along the upper margin and the 

 readings of the micrometer along the side, as depicted in 

 Plates III., IV., and V. With the various readings on which 

 the measurements and positions of these bands on the chart 

 depend, I need not trouble you, suffice it to say that on 

 shading the spaces bounded by the series of measured 

 points respectively constituting the sides of these bands, 

 the representations depicted in Plates III., IV., and V. were 

 obtained. My instrument was good, and the measure- 

 ments carefully made, and they may be safely accepted as 

 of very considerable accuracy. We may, therefore, treat 

 the representation as if it were the phenomenon itself. On 

 drawing lines along the sides of the band marked A, in 

 Plate III., they converge to a point below the scale, and on 

 drawing a line at right angles to the scale it cuts it at A '571. 

 The band B, treated in the same way, gives a primitive line 

 at X '540. These results are recorded in the primitive 

 spectrum, placed for purpose of comparison and convenience 

 of construction below the above spectrum. 



Some of the blood solution was next removed from 

 the hollow prism, about two-thirds, and the prism filled 

 up with distilled water. A considerably diluted solution 

 was thus obtained. On observing the spectrum, the result 

 represented in Plate IV. was seen, and recorded on a 

 similar chart in the same way as in the previous case. In 

 appearance generally, and in the form of the absorption 

 bands, considerable alteration is apparent. On ascertaining 

 the position of the primitive lines, A is found to occupy a 

 position on the scale at A -573 and B at X '542. It will be 

 observed that the difference is only in the third decimal 



