the Media of the Eye. 179 



curved at the other in the form of a sphere. A longitudinal 

 slit was made in this spherical curvature, and upon this curva- 

 ture the cornea was fastened. On the side of the cylinder was 

 a tube, which by means of an India-rubber tube was connected 

 with a bladder full of air. In this manner the cornea was always 

 kept stretched,, and the individual zones radiated through it 

 sharply denned. 



The aqueous humour had always to be obtained by a prick in 

 the fresh eye ; because the humour, as soon as it was poured out 

 from the interior by removing the crystalline lens, always showed 

 a slight turbidity from the injured pigment. 



The crystalline, finally, was not used in its natural con- 

 dition, owing to the difficulty of determining the focus for the 

 dark rays. The experiments with the crystalline were not 

 made with the rock-salt prism, but only afterwards, when the 

 rock-salt prism was replaced by a flint-glass one, when the lens 

 was pressed between two glass plates so that the substance had 

 a thickness of 2 millims. 



The first experiments, made in the summer of 1859, excluded 

 all use of glass. Rock-salt prisms and plates were the only 

 bodies which, excepting the media of the eye, were traversed by 

 the thermal rays. 



The reflecting galvanometer gave deflections in the red zone 

 which varied between six and eleven divisions of the scale. It 

 is not the object here to find the quantity of heat absorbed by 

 each medium of the eye, but only the ratios in which the various 

 thermal colours traverse the medium in question; hence the 

 number 10 has been chosen for the quantity of the red pencil 

 which has traversed any of the media, and the deflections, as well 

 as the quantities of heat, are reduced to the same unit*. 



On account of the smaller dispersive power of the rock-salt 

 prism as compared with the flint-glass prism used in the earlier 

 experiments, in the first experiments on the diathermancy of the 

 cornea every two zones were taken together to exert their heat- 

 ing influence on the thermo-pile. The following relations were 

 obtained for the quantities of heat of the different zones which 

 passed : — 



Zone, Violet and Indigo . . . 0*9 



„ Blue and Green .... 3*6 



„ Yellow and Red .... ]00 



First and second dark zone . . 3*7 



Third and fourth dark zone . . 0*8 



For the aqueous humour, which was in a layer of about 4 mil- 

 lims. thickness between two transparent rock-salt plates, the 



* Poggendoiff's Annalen, vol. lxxxix. p. 526. 



