EFFECTS OF RADIANT ENERGY ON THE EYE. 733 



about 0.01 square mm. as determined from photographic data, so 

 that for a long fixation the energy received would be distributed over 

 practically 5 square mm. of area. The density in the image as actu- 

 ally obtained in a long fixation would amount to about 20,000 ergs 

 per second per square cm., only about 200 P^^* of the energy density 

 which produced the positive result in experiment No. 53. This 

 difference is still further enhanced practically by considerable differ- 

 ence in the size of the image areas, the smaller one being relatively less 

 effective than the large on account of the more rapid dissipation of 

 heat. It is therefore evident that an exposure of even 12 minutes 

 with close fixation to a source as powerful as this arc, a thing which no 

 rational human being would be likely to undertake, still involves only 

 a small fraction of the minimum energy known to produce definite 

 lesions. 



Further evidence of the harmlessness of such exposures even to 

 very pow^erful artificial illuminants may be derived from noting that 

 the heat concentration in this case, that is 20,000 ergs per second per 

 square cm. is less than 3^, of that received by direct fixation of the 

 sun with the same pupillary aperture (see page 719). Now it is 

 well known that one can fix the sun for a very few seconds without 

 danger of anything more than a temporary scotoma. From this we 

 may conclude that the arc here considered may be fixed, as well as it is 

 possible to secure fixation, for a considerable period without running 

 danger of a permanent scotoma. 



In order to demonstrate beyond any possibility of doubt that the 

 retina cannot be injured by exposure of the eye to a source such as 

 the magnetite arc, one of us has actually fixed his eye upon this arc 

 at a distance of two meters for 6 minutes on two occasions. The 

 macula itself was not fixed upon the arc but upon an object of the 

 same size placed 2.6°, to one side of it. In this way more perfect 

 fixation of the image of the arc on the retina was ensured than if the 

 arc had been looked at directly. The results were only of a tempo- 

 rary character. There was considerable xanthopsia lasting about 

 3 minutes. The scotoma had disappeared after each observation in 

 less than ten minutes and could only be detected for a few hours by 

 careful dark adaptation of the eye. The following day no traces of it 

 were determined. It is therefore clear from actual experiment as well 

 as from general principles that even the extremely powerful arc which 

 we have here used is from the standpoint of injury to the retina entirely 

 harmless under any circumstances conceivable in practical use. Some 

 large carbon arcs may yield even two or six times the total heat energy 



