306 J. RHEINBERG ON BLACK AND WHITE DOT PHENOMENON. 



Theoretical considerations also teach us that when the dark 

 dots give place to white dots, there must be an equivalent amount 

 of darkness spaced between them. It follows the law of dis- 

 tribution of energy, which tells us that we cannot destroy any of 

 the light, we can merely rearrange it in other ways. 



In most high-power photographs of fine diatom structures, as 

 well as in visual observations, the extra bright space surrounding 

 a dark space, or vice versa, can be seen or detected.* 



(6) A second objection which has been suggested is that we 

 are scarcely justified in applying the ordinary laws of geometric 

 optics to such small structures, the dimensions of which may be 

 counted by one or two wave lengths, or even fractions of a wave 

 length of light. 



This objection assumes a sort of sui generis difierence 

 between the phenomena of the large and the small, and it may 

 well be pointed out that the whole tendency of microscopic optics 

 at present seems to be to break down such barriers. The fact is, 

 all optical appearances are necessarily complex, and due to 

 various causes. We cannot absolutely isolate one cause and 

 eliminate all the others. What does happen is that, whilst all 

 the causes are always at work, when the conditions are varied, 

 one cause becomes more prominent than another, and may 

 overpower the result of the others. Diffraction furnishes one of 

 the chief examples of this. It is ever present in an optical 

 image. Its eflfects gain or lose prominence according to size of 

 structure, angular aperture, and other reasons. Under certain 

 conditions it greatly disturbs a correct appreciation of the 

 image, overpowering the concomitant effects due to refraction, 

 reflection, etc. I have already pointed to some reasons why 

 diffiaction cannot be the direct cause of the black and white 

 dot appearances. Unless, therefore, some new and unknown law 

 is causing the effects, for which we have no evidence whatever, 

 we are left with those causes which we do know are at work, and 

 of which the laws are well known. 



(c) It is interesting to note that both the black and white dot 

 appearances with light and dark areas respectively are well 

 shown in photographs of the very finest secondary structure in 



* Care must be taken that the light is not too strong, because the eye is 

 considerably less sensitive to differences of brightness, as compared to the 

 field, in a very strong light than in a light of moderate intensity. 



