732 VERHOEFF AND BELL. 



possible to obtain even this eflfect except through strong focussing of 

 the hght upon the eye, a condition which is not found in the use of 

 ordinary illuminants for any purpose. So far then as infra red 

 radiation is concerned the eye is not subject to any special dangers, 

 and the concentration of heat upon it in any way would inevitably set 

 up a danger signal of painful sensation long before any definite heat 

 effects could be obtained. 



In fact it is easily demonstrable that the full radiation that can 

 under practical conditions be received from the most powerful illumi- 

 nants is incapable of producing on the retina any lesions due to 

 thermic action such as may be found in eclipse blindness. In our 

 Experiment 53, already referred to, changes in the pigment epithelium 

 and a definite burnt area were produced. The diameter of the area 

 in which histological changes were clear was 3 mm. shading off toward 

 the edges and having a central area about 1 mm. in diameter, in which 

 the damage was serious and comparable with that in eclipse blindness. 

 We have found that in this case the concentration of the energy in 

 the image amounted to very nearly 4.2 X 10® ergs per second per 

 square cm., roughly 2g of that found in the solar image formed directly 

 through a 3 mm. pupil. The exposure to this intensity lasted 12 

 minutes. Here then is a definite case in which the result was positive. 

 Negative results, however, of which a few were obtained in our experi- 

 ments, are unsatisfactory since they do not take account of the wan- 

 dering of the image, or of imperfect fixation which as we have shown 

 would be likely to avert injury as in the typical case of intermitted 

 exposure to heat. 



Our large magnetite arc gives at a distance of 2 meters a total 

 radiation of about 2500 ergs per second per square cm. At this dis- 

 tance from the arc we find the pupil of the human eye is narrowed to a 

 scant 2 mm. and this area would intercept about 75 ergs per second 

 of the total amount stated, allowing 5 absorption as heretofore. 

 This means that the energy concentrated on the retina would be about 

 50 ergs per second. To compute the energy density requires a knowl- 

 edge of the size of the image including the element of imperfect 

 fixation. We have found from personal experiments that on fixing 

 at 2 meters the magnetite arc for a few seconds and measuring the 

 size of the scotoma produced by viewing at the same distance a card 

 ruled to centimeters, that even for this short fixation period the area 

 of the scotoma is nearly four times that of the geometrical image of 

 the source. On fixation of 6 minutes the scotoma rises to twenty-five 

 times the size of the geometrical image. This latter has an area of 



