±03 COLOR PHOTOGRAPHY. 



to that passing directly, a phase change of a half wave length, and 

 since at each of the two elementary mirrors it receives the same phase 

 change, the phase change on reflection at an elementary mirror is one- 

 fourth wave length. In this the phase change is reckoned with refer- 

 ence to a ray reflected without phase change from the plane at the 

 middle of the elementary mirror. 



The result above described will be deduced in another way in the 

 connection mentioned on page 184, and difficulties and objections 

 encountered will also be discussed. 



All that has just been said concerns the case when there is no absorp- 

 tion. Such a case is furnished by the chro in -gelatin process of 

 Lippmann, 1 in which the transmitted colors are complementary to those 

 reflected. 



When absorption is present it would readily be decisive in the case 

 of transmitted light, because each complementary transmission color, as 

 in the colors of thin plates, must contain much white light and be 

 therefore faint. 



Thus Krone 2 was able to observe only the characteristic color of the 

 precipitate formed by development in Lippmann's haloid-silver plates 

 by transmitted light, and I have myself made the same observation. 

 Lippmann himself says that in two silver bromide-albumin plates he 

 observed complementary colors transmitted. 3 The absorption must in 

 this case have been very slight. 



Where the same colors appear by transmitted as by reflected light, 

 these can not be due to interference, but must be caused by absorption. 

 Conversely, absorption, when it is not too strong to show surface colors, 

 must show the same colors by transmitted as by reflected light, for this 

 is nothing but doubly transmitted light. 



We have thus a second proof that the colors in Seebeck's and Poite- 

 vin's processes are body colors. 



XL — The Cooperation of Body Colors in Becqtterel's 



Process. 



I remarked in the general survey (I) that it would be astonishing if 

 Seebeck's plates showed body colors under colored illumination and 

 the plates of Becquerel, which chemically are almost identical, failed 

 to show them. It was, however, to be expected that these body colors 

 Avould be hard to recognize so long as interference colors were very 

 strong. It is not difficult to suppose that these latter would be weak- 

 ened by a long time of exposure, in consequence of which the photo- 

 graphic action must penetrate very close to the vibration nodes of the 

 stationary waves. This result was observed by Krone 4 with Lippman's 



1 Lippmann. Couiptes rendus, 115: p. 575, 1892. 



2 Krone. Darstelling der natiirlichen Farben, p. 54. 



3 Lippmann. Comptes rendus, 114: p. 9(52, 1892. 



4 Krone. Deutsche Pkographen Zeitung, p. 187, 1892, edited by Valenta. 



