112 



PHYSIOLOGY 



CHAP. 



iii which Hiifner has determined the absorption ratio* or constants for, viz. : 

 0,001330 and 0,001000 respectively. 



The limits of these spectral positions, which are comprised between the 

 Fraunhofer D- and E- lines, may be expressed in wave-lengths by means 



FIG. 37. U', Micrometer screw, divided into hundredths, each turn of which displaces the index of 

 the scale S by one division. This serves to regulate and measure the widtli of the slit /, which 

 can be carried by a horizontal movement to the centre of the eye-piece. /,-, Micrometer screw 

 divided into hundredths. each turn of which displaces the index of a scale n by one division. 

 This moves the eye-ptece by an angular development to carry it to any given region of the 

 spectrum. 



of the table published by Kriiss (as above cited). In practice they can be 

 obtained by finding 011 the illuminated scale of the spectrum (Fig. 36, s) 

 the values 'corresponding to the two wave-lengths calculated, and limiting 



the spectral region which these 

 comprise by the horizontal screw 

 of the telescope (Fig. 37), and that 

 marked W, which controls the 

 slit / of the eye-piece. The ab- 

 sorption chamber A is then placed 

 on its support between the plane 

 of the spectrum and the source of 

 light, taking care that the upper 

 surface of the cube D corresponds 

 exactly with the line of division 

 between the first and second slit, 

 and that the aperture of these 

 corresponds to a complete turn of 

 the screw: a turn divided into 

 100 parts, as shown on the scale 

 affixed to the screws v and v' in 

 Fig. 38. 



On then looking through the 



Fl( V 3 S'I^' *"' Mic . rom ter screw divided into instrument, two positions of the 

 hundredths, serves to widen or narrow the slit /, /', -11 i -T.I 



by simultaneous displacement of the two plates Spectrum Will be Visible, Olie below 



that confine it. the other : one is brighter, corre- 



sponding to the cube of Schultz, 

 the other obscured by the absorption due to the solution of oxy haemoglobin. 



The slit corresponding to the brighter part of the spectrum is then 

 narrowed until it assumes the same tone of light as the other, and the 

 scale on the screw read to show how many turns were required to produce 

 uniform obscurity. From this number, which indicates the intensity of the 



