1912.] The Effects of Ultra-Violet Rays vpon the Eye. 



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resulting photograph, and the precautions taken quite excluded the possibility 

 of stray light coming in between the cell and the slit. 



2. A thin layer of lens substance was squeezed out flat between the 

 parallel sides of the cell. This was done to eliminate any possible apparent 

 absorption due to refraction in the lens. 



Visible. 



A preliminary series of spectrophotographs were taken to eliminate 

 absorption within the limits of the visible spectrum. 

 Source of Light. — The crater of a carbon arc. 



Instrument. — A two-prism spectroscope, prisms and lenses being of glass. 



Plates. — Commercial " orthochromatic," which regularly showed three faint 

 absorption bands in the green, yellow, and red, representing the absorption 

 of the dye used to sensitise the plates to red light. 



Result. — All the media were found to be uniformly permeable to rays 

 between the wave-lengths 660 fj.fj.-S90 ilil. (The visible spectrum extended 

 approximately 760 fu.fj.-380 fifi) (Plate 4). 



Ultra-violet. 



For the ultra-violet rays the iron arc was the source, and quartz lenses 

 and prisms were employed throughout. Ordinary plates were used, i.e. plates 

 containing no dye and hence giving no absorption bands. 



The range of wave-lengths covered in this series is 450 fifj, to 230 fj.fi. 



Cornea. — The cornea offers no resistance to rays of wave-lengths longer 

 than 295 fj.fi, but all those beyond this limit are completely cut off (Plate 5, B). 



Lens. — (a) Suspended in normal saline. Eays of wave-lengths less than 

 350 fifj. are absorbed completely. The line is not a sharp one, the absorption 

 commencing about 400 fj.fi (Plate 5, F). 



(b) Squeezed to different thicknesses. The absorption varies pari passu 

 with the thickness of the layer of the lens substance (Plate 5, C, D, E). 



It is clear, therefore, that between wave-lengths 400 /ifi and 300 fi/i there 

 is a group of rays to which the cornea offers no resistance, but which are 

 absorbed by the lens, and are consequently capable of inducing changes in it. 



Vitreous. — The vitreous, in a layer 3/16 inch thick, shows a broad 

 absorption band, extending from 280 fifj, to 250 fifj., with a maximum at 

 270 fj.fi. The margins of the band are ill defined (Plate 5, H). 



These results agree closely with those of the observers referred to below. 

 The shortest interval between the death of the animal and the taking of the 

 observation was 3 minutes, the longest 11 minutes. Observations were 



