186 Professor R. W. Wood [May 19, 



difference to be noted in these pictures is that the flowers in the 

 garden, which are white in the picture taken with visible Hght, 

 disappear entirely in the picture taken by means of the ultra-violet 

 radiation. The white garden flowers become almost black as is shown 

 in Fig. 5, which shows white phlox photographed by visible and 

 ultra-violet light. It occurred to me that this ability of the white 

 flowers to absorb the ultra-violet rays might play some economic part 

 in the growth of the plant. I therefore experimented with some 

 flowers which had been grown under glass, and had thus been 

 deprived of ultra-violet, but I was unable to find any marked differ- 

 ence between those which had been grown in the open and others 

 which had been deprived of their full quota of this radiation. It is 

 possible that if the experiments were carried on through the course 

 of a number of generations we should find a difference. I have 

 found, however, that all white flowers are not equally dark when 

 photographed with ultra-violet light. White geraniums, for example, 

 come out much lighter than common white phlox, which is practically 

 black when photographed through the silvered quartz lens. 



In order to demonstrate the difference in the appearance of one of 

 the common pigments when viewed respectively with visible light and 

 with ultra-violet radiation, some letters were painted in Chinese white 

 on a page of a magazine. In the photograph (Fig. 6) taken with visible 

 light the Chinese white appears as white as the paper itself, if not 

 indeed whiter ; but, photographed with the ultra-violet radiation, it 

 comes out absolutely black. One may say that what is Chinese white 

 in visible light becomes Japan black in ultra-violet. Under this 

 radiation also black printer's ink becomes lighter than in visible light. 

 This failure in the reflecting capacity of Chinese white is a source of 

 some annoyance in reproducing drawings executed in part in this 

 medium, as has been pointed out by Mr. A. J. Newton. In working 

 with my Chinese white I made a mistake in one letter in the word 

 "appears," and carefully wiped it out, leaving no trace of the cor- 

 rection discernible in visible light ; but when the photograph was 

 made with the ultra-violet, the erasure, otherwise invisible, showed as 

 a black smudge. The ultra-violet camera is evidently very much 

 more sensitive than the eye to the presence of traces of Chinese white 

 on the printed page, for so far as I could see every particle of the 

 pigment had been removed. Whether this has any bearing upon the 

 detection of forgeries has yet to be discovered. 



Another class of work in which this comparative study is likely to- 

 be of service is the photography of celestial bodies. For the full 

 moon the exposure through the silver screen was two minutes with 

 ultra-violet light belonging to the region :^000 to ;->200. This length 

 of exposure necessitated an equatorial telescope with some means of 

 driving it to compensate for the moon's movement. The support for 

 my telescope was the framework of an old bicycle minus tlie wheels. 

 This carried a 4-inch refractor and a quartz-silver telescope, and by the 



