EFFECTS OF RADIANT ENERGY ON THE EYE. 771 



effect of thickness or intensity before and after transit shows a varia- 

 tion possibility of small degree thus, — 



Ji = Jo-e'^'^ where Ji = intensity after transit 

 Jo = intensity before transit 

 d = thickness and k = coefficient constant. 



The human lens he found to vary considerably, as will be shown later, 

 with such factors as age, consistency and color. 



As has been stated the absorptive power of the lens and other eye 

 media for ultra violet rays, and the limit of visibility of the spectrum in 

 the ultra violet region are two problems whose investigation has been 

 carried forward in the same stages since it was obvious from the start 

 that the determination of one would throw much light on the other. 



Brucke^^ really opened the subject in 1845 when he speculated as 

 to the range of the visible spectrum and the reason for the invisibility 

 of the ultra violet rays. In the manner described he found the ox 

 lens to absorb rays below 370 mm- Bonders ^^, using the method of 

 fluorescing screens of quinine sulphate, discovered by Stokes, at- 

 tempted to measure the absorptive power of the lens but the glass 

 containers vitiated his results. 



Stokes ^^'^ by direct observation of the solar spectrum through a 

 quartz prism, thought he could see as low as the 372 nfx, 358 ij-h, 

 and 335 mm lines and perhaps further. 



In the same way Helmholtz ^^^ with a quartz optical system could 

 see a few lines in the 372 mm and 318 mm region, although his eyes were 

 very myopic. He also observed the ultra violet rays directly through 

 holes in the fluorescing region of a screen of quinine sulphate upon 

 which the spectrum was thrown. 



By similar methods Listing ^^^ placed the limit of visibility at the 

 372 iJLiJL Hne and Sekulic ^^* at the 358 hijl line. Mascart 2*° however, 

 using high intensity of ultra violet illumination, considered lines as 

 low as 313 niJt, to be visible. Soret ^*^ by photographic methods found 

 the vitreous humor of the ox in 1 cm. thicknesses to have the same 

 absorptive power as the cornea of 294.8 fxix. He found that the lens 

 of the ox absorbs rays shorter than 383 iJ.fx, and the entire eye has the 

 same limit. Nevertheless he maintained that the human eye could 

 see rays as short as 294.8 iJ.fj.. 



Eisenlohr ^°^ threw doubt on these results when he pointed out that 

 fluorescence alters the ultra violet rays so that the observation by 

 means of or in the presence of fluorescent light, is not accurate. He 

 found even on white paper screens that fluorescence rendered rays 



