724 VERHOEFF AND BELL. 



arc by one of us the energy entering the pupil was of the order of 

 magnitude of 444,000 ergs per second. Examination of the retina 

 showed that the lesion produced was practically 3 mm. in diameter. 

 Hence the concentration of the energy in the image allowing the same 

 absorption as in the previous computation amounted to nearly 42 X 

 10^ ergs per second per square cm. This is roughly ^ of the concen- 

 tration in a direct solar image and correspondingly the lesion produced 

 was comparable with that of a typical case of eclipse blindness. It is 

 quite impossible to get an accurate idea of the critical length of fixa- 

 tion which appears in cases of eclipse blindness, since the observations 

 producing it are generally discontinuous and not noted. This ex- 

 periment, however, indicates, making due allowance for the extent 

 of the experimental image and for the extremely small size of the 

 pupil in looking at the sun with the naked eye, that the critical period 

 for the development of eclipse blindness is, with close fixation, of the 

 order of magnitude of a minute or less. An exposure of even a few 

 seconds would be highly dangerous were it not for the extreme miosis 

 set up and the usual wandering of the image upon the retina. 



Rapid shifting of the focal image on the retina gives the tissue an 

 opportunity for cooling, so that if the fixation at a single point is not 

 long enough to produce destructive effects little permanent damage 

 can be done, although the scotomata may be severe. Our experi- 

 ments Nos. 100 and 101, in which the exposure to the solar heat 

 through a blue uviol screen was intermittent, show this excellently. 

 In the first the exposure was for alternate seconds over a period 

 aggregating ten minutes, or more than six times as long as necessary 

 to produce burning of the retina in a continuous exposure. No 

 damage was done. The second experiment, in which no water cell 

 was used, consisted of 220 exposures of ^ second with one to three 

 seconds interval between. In this case there was again no damage 

 done although the exposure was three and two-thirds times as long as 

 was required to produce destructive lesions of the retina in two differ- 

 ent cases with a continuous exposure. 



In this connection one may note that the experiment of Best^^ in 

 fixing the sun for ten seconds through a screen of blue uviol glass was 

 a somewhat hazardous one since this glass lets through a very mate- 

 rial proportion of the energy from a high temperature source like the 

 sun. Best's purpose in making this experiment was to show that 

 ultra violet light is not injurious to the retina of a normal eye. The 

 exposure however, was too brief for the result to be of importance in 

 this regard. It is not ultra violet energy which is to be feared in a 



