i t Z WALKER. 



visible as low as 354 \xix in length while observation of the same light 

 through the spectroscope showed no rays visible less than 395.6 \x\i in 

 length. Later Sauer ^°^ using metal electrodes came to the same con- 

 clusion. 



With the use of the arc light de Chardonnet ^^ photographed the rays 

 able to pass through the human lens and found absorption began at 

 the "H" line (397 /x/x) increased to the "L" line (381 mm) ^.nd became 

 total at the "M" line (372 \x\i). The absorptive power of the cornea 

 lay between the 304 /x^i and 299 mx lines and for the vitreous humor 

 between 304 ix\x and 310 \x\x. To de Chardonnet belongs the credit of 

 properly emphasizing the significance of the higher absorption of the 

 lens in determining the lower limit of visibility of the spectrum. He 

 concluded that patients having clear media after cataract operations 

 could see more of the spectrum than normal eyes. By thinly silvering 

 a quartz glass plate he was able to prevent all except rays below the 

 343 ix^x line from passing through so that when normal eyes attempted 

 to observe an arc light through the plate it was entirely invisible. He 

 found two aphakic patients however who could tell when the arc 

 light was turned on and off or when it was moved while lighted. 



Widmark*^^ next took up this question in a very thorough manner. 

 He used Hasselberg's modification of Rowland's spectroscope using a 

 grating with a radius of 1.6 meters. The light source was an arc light 

 with iron poles. The discontinuous spectrum obtained in this way 

 gives sharp well known lines to examine and is free from aberrant light. 

 Widmark examined eight aphakic patients ranging from 59 to 68 years 

 of age. Seven could see lower in the scale than he, himself, could. 

 Four of these were examined roughly by observing the spectrum 

 thrown on a screen. One of these could see no better than himself 

 so that the above more accurate method was used on the second four 

 giving the following results for the ultra violet limit, — 313 /x/x, 313 /xju, 

 342 MM and 344.5 mm- 



In order to test the normal range at various ages for comparison 

 he examined 59 individuals ranging from 11 yrs. to 74 yrs. of age. 



The results are here tabulated. 



