NOTE 



ME. SOEBY'S SPECTEUM ANALYSIS OF THE BLOOD. 



IN addition to the observations made iu the text, it may be remarked that 

 the scale of measurement adopted by Mr. Sorby is obtained by means of two 

 small Nicol's prisms and an intermediate plate of quartz. Mr. Sorby ob- 

 serves in his paper, contained in the " Proceedings of the Royal Society," for 

 1867, that if white light passing through two such prisms without the plate 

 of quartz be examined with the spectrum microscope, it of course gives an 

 ordinary continuous spectrum, but if a thick plate of quartz or selenite be 

 placed between the prisms, with the axis of 45 to the plane of polarization, 

 though no difference can be seen in the light with the naked eye the spectrum 

 is entirely changed. The light is still white, but it is made up of alternate 

 black and colored bands, evenly distributed over the whole spectrum. The 

 number of these depends upon the thickness of the depolarizing plate, so 

 that we may have if we please almost innumerable fine black lines, or fewer 

 broader bands, black in the centre and shaded off at each side. Hence, as 

 the number of divisions depends on the thickness of the interference plate, 

 it became necessary to determine what number should be adopted. The 

 number 12 was selected for the number of divisions purely for the sake of 

 convenience. This number is easily counted, and the sodium line D comes 

 very accurately at 82. The centre of the bands is black, and they are 

 shaded off gradually at each side, so that the shaded part is about equal tp 

 the intermediate bright spaces. Taking then the centres of the black bauds 

 as 1, 2, 3, etc., the centres of the bright spaces are H, 2s, 3-}, etc., the lower 

 edges of each 5, 1!|-, etc., and the upper IT, 2], etc. We can easily divide 

 these quarters into eighths by the eye, and this is as near as is required, cor- 

 responding as nearly as possible to jo^th part of the whole spectrum visible 

 under ordinary circumstances by gaslight and daylight. Absorption-bands 

 at the red end are best seen by lamplight, and those at the blue end by day- 

 light. On this scale the position of some of the principal lines of the solar 

 spectrum is about as follows : 



A . . . f B...l}...C...2f D . . . 3J 



E ... -6ft b ... 6^ ... F ... 7^ G . . . 10| 



Mr. Sorby further remarks, that in order to describe spectra he has devised 

 a simple notation, employing types in constant use, by which it is easy to 

 give a single line in all the essential particulars that would otherwise require 

 a long and tedious description, or a number of drawings and woodcuts. The 

 intensity of the absorption is expressed by the following types : 



Not at all shaded, Blank space. 



Very slightly shaded, ... Dots with wide space. 



Decidedly sliaded, Dots closer together. 



More shaded, Very close dots. 



Strongly shaded, but so that a trace "| ,, 



of color is stiil seen, . . . .} Three hyphens close. 



Still darker, Single clash. 



Nearly black, Double dash. 



