

Absorption Spectra. 235 



error in their construction, and, since the position of the 

 absorption bands represented in Plates II., III., IV., and V.. 

 were determined from them, and the primitive lines from 

 these, such error would affect the result. In Plate VI. 

 the primitive spectra of arterial blood (human), obtained 

 by examining wedge-shaped portions of the medium 

 under the most extreme conditions of variation as regards 

 strength, angle of prism, dispersion, and instrumental 

 conditions, are represented together under the same dis- 

 persion scale. From this it is seen how close the agree- 

 ment is, in the most extreme instance, the variation being in 

 the third decimal place only, whilst in other instances they 

 are identical, or almost so. With special arrangements for 

 indicating the position of the edges of the bands, and 

 more perfect forms of wedge-shaped absorption cells, I 

 believe the results would be still more uniform. 



In order therefore to remove these sources of error, and 

 also to facilitate this mode of observing, I am having con- 

 structed, and hope on a future occasion to bring before your 

 notice, the following two simple appliances : — 



Filar Eye-piece. This is an eye-piece with a circular 

 disc of glass fixed in the focus of the eye-lens. A vertical 

 line is ruled through its centre, and at right angles to this, 

 at a distance of one-third from its lower end, a trans- 

 verse line, which is divided by a series of fine vertical lines. 

 A pair of webs, moveable, so as to allow of their being laid 

 along the edge of an absorption band, completes the ar- 

 rangement. The appearance seen on looking through the 

 eye-piece of the spectroscope is depicted in Plate VII., Fig. 1, 

 arranged for centring the telescope on the primitive line of 

 a converging or wedge-shaped band. Extending from the 

 upper edge at A, through the centre to the lower edge at B, 

 is the vertical line. From the lateral edge at C, and cutting 

 the vertical line a little below its centre and extended to D, 

 is the transverse line, which is seen divided by vertical 



