766 WALKER. 



in 1888 to divide the large number of reported cases into two classes; 

 a mild group without retinal disturbances and of good prognosis, 

 and a severe group with retinal disturbances and of bad prognosis. 

 However, after the classical experiments of ^Yidmark these theories 

 and classifications were no longer found to be useful. Widmark*^^ 

 in 1889 exposed the rabbit's eye to various parts of the arc light spec- 

 trum. He found that when a 1200 c. p. arc light was used for 10 

 min. on the rabbit's eye without screening out any ultra violet rays 

 all the typical symptoms of electric ophthalmia appeared after a 

 latent period of 6 hours. By varying the time of exposure any degree 

 of injury could be produced from a mild erythema to ulceration of the 

 conjunctiva and cornea. But if a common glass plate 0.5 to 1.0 cm. 

 thick was interposed the rabbit was entirely protected. Thus he 

 established for the first time that rays below 300 /XM in length were 

 chiefly responsible for the outer eye trouble. This particular point 

 was confirmed by Ogneff ^^^, Hess ^'^'^ , Kiribuchi ^°^ and subsequently 

 by practically all observers. Further ^Yidma^k concluded that ultra 

 red and the visible rays are entirely without effect, outside of common 

 heating effects. 



Widmark made another important contribution to the knowledge 

 of this subject when he drew attention to the striking resemblance 

 of ophthalmia electrica and the disturbance found on the outer eye 

 in cases of snow-blinding. He showed that they both had the same 

 latent period and were ushered in with the same syndrome of symp- 

 toms. Further that erythropsia, temporary blindness, or blind spots 

 occurred in both. The fact that ultra violet light is stronger on high 

 mountains and snow covered surfaces as had been shown by Tyndall 

 and subsequently verified by Hehnholtz, Langley, Cornu and Mascart, 

 was a further argument emphasized by Widmark in support of his 

 contention that ultra violet rays are responsible for both ophthalmia 

 electrica and snow blinding. 



Birch-Hirschfeld, Hertel, Best and others had up to 1907 found a 

 thick plate of common glass to be sufficient protection from electric 

 ophthalmia as Widmark had pointed out. But in 1907 Stockhausen 

 after one half hour working with arc lights received a severe ophthal- 

 mia electrica through glass protection. Schanz & Stockhausen ^^° 

 therefore repeated Widmark's experiment and found common glass 

 to be inefficient protection for long intense exposure. They were 

 able to produce the characteristic symptom in a rabbit's eye through 

 18 mm. of common glass after 4 hours' exposure to a 15 amp. arc light. 

 Thus stimulated they studied the manufacture of glass carefully, 



