774 WALKER. 



to Tigerstedt^^^ it is generally accepted that the retina is sensitive 

 to such ultra violet rays, as are able to penetrate the eye media and 

 be focussed upon it. 



The same does not hold for ultra red rays although Helmholtz 

 thought it did from the work of Briiche and Knoblauch ^^ in 1846 

 who found that heat from the Argand burner did not penetrate the eye 

 appreciably. Cima '^^, 1852, also Janssen ^^^, and Franz ^^^ in 1862, 

 found only about 9% of the heat from a Locatelli lamp was trans- 

 mitted. This was confirmed by Klug ^°* in 1878 with gas and sunlight. 

 Tyndall ^''^, 1865, with a 650 c. p. arc lamp found that about \ of the 

 dark heat rays were transmitted through the vitreous of the ox. 

 Engelman ^^° in 1882 using the bacterium photometricum, — which 

 always migrates to the infra red region when exposed to the spectrum, 

 as an indicator, found the same phenomena when water glass vitreous 

 lens or cornea was interposed. Hertel ^^^ in 1911 showed the lower 

 limit of subjective and of objective stimulation of the retina were 

 about the same, lying between 820 nn and 840 mm- Vogt *°^ however, 

 in 1912 showed conclusively that a great amount of the ultra red light 

 reaching the retina is not visible, in fact as much as 80% or more. 

 Further on normal human eyes he found that 3% of the heat reached 

 the retina and less than 1% passed on into the orbit. 20% to 25% 

 passed through cornea or sclerotic. The aqueous absorbed 20-30 % 

 of the heat transmitted by the cornea. The cornea iris and lens 

 together transmit 6% of the heat falling on the cornea. The lens 

 absorbs 30% of the heat transmitted by the cornea and iris. Vitreous 

 absorbs nearly 60% of the heat falling on its anterior surface. The 

 upper lid transmits 6%. 



Fluorescence has long been a subject of much interest and study. 

 A. von Graefe knew that fluorescence of the lens was due to ultra 

 violet light and Helmholtz ^^° after an extended study of the fluores- 

 cence of the lens, quinine sulphate solutions and other fluorescing 

 bodies, concluded that fluorescence in general is due to the appearance 

 of rays of various length, and is therefore really mixed or white light. 

 Fluorescence was then the result of a transformation of ultra violet 

 rays to rays of greater wave length. He considered the rays between 

 400 ixjx and 300 ixfx, to be chiefly the ones transformed. Widmark 

 noted an apparent decrease of fluorescing power of the lens as the age 

 of the individual increased and the absorptive power increased. 



Schanz and Stockhausen ^^^ in 1909 took up the question of fluo- 

 rescence in connection with their study of the properties of " Euphos- 

 glas" which they found in certain grades to absorb rays below 400 /x/x 



