480 



MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



Eed, orange, yellow, and green afford nearly as mucli protection against insolation as complete 

 shadow. Blue and violet give niuoli less protection, and if the insolation continues for four or 

 five hours all the colonies are killed. In the articles referred to in the literature collected on this 

 subject the authors attribute this protective power to the opaqueness of red, orange, yellow, and 

 green to the actinic rays, while blue and violet are perfectly transparent to these rays. Glass 

 affords some protection from the sun, l)ut as a constant tactor in this experiment it does not alter 

 the result as far as comparison of the effect on the different organisms is concerned. 



If during insolation the sun becomes obscured for even a short time, as from wind clouds, the 

 effect is seen by diminution of the number of colonies destroyed. 



Diffuse light has very little destructive effect upon the organisms studied. 



Gas and electric light as used in the illumination of houses have little if any effect upon 

 these organisms. 



Under ordinary circumstances one might be tempted to explain the effect of sunlight as a 

 disinfectant as produced by the drying of the substances exposed to it, especially in the case of 

 those bacteria which do not form spores: but desiccation for a considerable period of time has 

 little effect upon the bacillus of typhoid or the colon bacillus, neither of which is known to 

 form spores. 



Geisler (Zur Fratje iiber die Wirhung des LicMes auf BaMerien, Centrnlhl.f. BaltcrioL, Jena, 

 1802, XI, p. 161) reports that he found no qualitative difference between the effects of the electric 

 arc light and the direct sunlight; but that sunlight has a greater effect in checking the develop- 

 ment of the bacillus of typhoid; that all the rays of the spectrum, except the red, exercise some 

 inhibitory effect; that the heat of the sun's rays has some effect, and that perhaps changes in the 

 culture medium must be taken into account. 



Light affects some of the bacteria which it does not destroy. For example, it diminishes the 

 ]iignient-producing power of some of the chromogenic bacteria, such as the bacillus jn/ocyancns 

 and the hncillus jrrodiiiiosKS, and it probably lessens the pathogenic power of such organisms as 

 the bacillus of typhoid and the bacillus of tubercle, because it not only diminishes the raijidity of 

 their multiplication and hence the amount of their products, but so modifies them that, for a time 

 at least, when placed in the dark or inoculated into an animal the siiccessive generations are less 

 vigorous and prolific and give rise to less of their characteristic jiroducts. Some bacteria seek 

 the light and some i^roduce light, especially some forms resembling the cholera spirillum. 



The practical importance of sunlight in preventing disease and destroying contagion has long- 

 been known, but it is only recently that we have obtained any definite knowledge as to how it 

 acts. Eecently Vou Esmarch [Uehrr Soiinen-Besinfection, Zeitschr. f. Hyy., XVI, 1894, 257) has 

 given x\\e results of a number of trials in disinfecting bedding, skins, etc., by sunlight, and while 

 the action of the light was, as might be ex^iected, mainly confined to the surface, yet it extended 

 more deeply into a iiillow containing the diphtheria bacillus than one would have anticipated. It 

 has been proposed to employ a i)owerful movable arc light to disinfect rooms, and it is possible 

 that good results may be thus obtained. The bacillus of tuberculosis appears to be more quickly 

 destroyed by light than some other forms, and is killed by diffuse daylight in about a week. 



The following tables and charts show the results obtained by Dr. Peckham. In the charts 

 each color is that of the glass used during insolation. Charts are given also to show i^ercentage 

 destroyed as comj)ared with the half protected by the different colors. 



Charl slioifiiig the protection from insolation giren htj Mack. 



