CHAP. II.] 



THE BLOOD. 



direct determination of the wave-length of any region in the visible 

 spectrum. 



The scale is similar to that shewn below, except that the position of the 



Fm. 19. 



SCALE OP WAVE-LENGTHS, SIMILAB TO THAT IN ZEISS'S SPECTROSCOPES. 



lines of Frauenhofer is not marked on the scale. The numbers attached to 

 the divisions on the scale indicate wave-lengths expressed in 100,000ths of 

 a millimetre; and each division indicates a difference in wave-length equal to 

 one hundred thousandth of a millimetre (0*00001 mm.). By the eye, the 

 position of any line situated between two divisions of the scale can be 

 estimated to one-tenth of a division, so that its wave-length can be ex- 

 pressed in millionths of a millimetre. In using Zeiss's instruments, the 

 observer commences by causing Frauenhofer's line D, or the sodium line, 

 to coincide exactly with that part of the scale which expresses its 

 wave-length, that is to say to correspond to division 58*9 of the scale 

 (which expresses a wave-length of 589 millionths of a millimetre or 

 0*000589). Having done this the scale is accurately set for all other 

 points. 



Let us suppose that the observer wishes to determine the wave- 

 length of Frauenhofer's line E. He will at once see that the line is placed 

 between divisions 52 and 53 and he would determine its position between 

 two divisions to within one-tenth of a division, but probably much nearer. 

 The reading would probably be 52*7, which would give for the wave- 

 length of E 527 millionths of a millimetre, a result which is only 

 three ten-millionths below the value deduced from the observations 

 of Angstrom 1 . As a result of many experiments with one of Zeiss's 

 spectroscopes the Author has found that the mean error in his case is not 

 greater than 0*000001 mm. 



Printed blank maps accompany Zeiss's instruments, which correspond 

 exactly to the scale of the spectroscope. There is therefore not the slightest 



1 The following are the wave-lengths corresponding to Franenhofer's lines A, B, C, 

 D, E, F, G-, according to the most recent measurements, expressed in millionths of a 

 letre : 



A 760-4 

 B 687*4 

 C 656-7 

 D 589-4 

 E 527*3 

 F 486-5 

 G 431-0 

 H 396-8 



