99 



VERHOEFF AND BELL. 



directed and kept in position. In use the mirror was slightly tilted 

 so as to throw the focus just out of the path of the direct incident 

 beam. The concentration of energy obtained by this instrument was 

 enormously great, owing to the size of the mirror and its relatively 

 short focal length. Its area was about 530 square cm. so that with 

 an average reflective coefficient of 0.75, with good sunlight the re- 

 flected energy would amount to some 4 X 10^ ergs per second. The 

 image of the sun formed by this mirror is 13.2 mm. in diameter or 

 about 1.36 square cm. in area. The energy at the focus then amounts 

 to approximately 30 watts per square cm. A pupil expanded to say 

 10 mm. b\' the use of mydriatics would therefore take in a pencil 

 equivalent to about 24 X 10^ ergs per second. Allowing as in other 

 cases one-third for the energy absorbed by the media of the eye as a 

 whole, the energy incident in the image would be approximately 16 X 

 10^ ergs per second. The diameter of the image in this case is just 

 over 2.5 mm., corresponding quite exactly to an area of 5 square mm. 

 The energy density in the retinal image therefore would be about 32 X 

 10^ ergs per second per sciuare cm. and it was found in our experiments 

 that an exposure of \ second to this intensity was sufficient to produce 

 a destructive thermic effect in the retina. This short period is very 

 striking in comparison with the relatively long exposures necessary to 

 produce typical eclipse blindness with the naked eye, although it 

 agrees very well with the data which we later cite regarding energy 

 burns from other sources. The secret of the relative resisting power 

 of the naked eye is that usually in observations of the sun, the pupil 

 is in extreme miosis, so that the amount of energy received is probably 

 not more than 6% of that computed for the normal pupil, while the 

 extremely small area of the solar image favors rapid dissipation of the 

 energy not found when a considerable area is attacked, as in the case 

 of the mirror experiments. The latter condition we have often noted 

 in thermal experiments of other sorts with the big mirror and lenses 

 of various kinds. A concentration of energy very much greater than 

 that from the mirror, acts much more sluggishly on inflammable 

 material when the focus is merely a minute point instead of an ap- 

 preciable area. Another factor that tends to protect the human eye 

 from the thermic action of light sources of small size, is the impos- 

 sibility of perfect fixation for any length of time. This is well shown 

 by some experiments on our own eyes (see page 732). 



The screens employed in the work are noted in connection with the 

 various experiments. Inasmuch as silver reflects very badly in the 

 region near the extreme ultra violet end of the solar spectrum and it 



