M. 0. Lea — Rontgen Rays not present in Sunlight. 363 



Art. XLII. — Rontgen Rays not Present in Sunlight / by 

 M. Carey Lea. 



If Prof. Pontgen's views as to the nature of the X-rays are 

 correct, it would seem that they ought to be found amongst the 

 many forms of radiant energy received from the sun, and 

 various observers have thought that they so found them. Some 

 experiments, the most important of which will be here briefly 

 stated, do not seem to support this opinion. 



1. A very sensitive dry plate (S. 27) was placed between the 

 leaves of a book so that 100 leaves and the red paper cover 

 should be between the sensitive film and the sunlight. The 

 book was then packed in a box frame to exclude all light from 

 the sides. A large and thick lead star was then fastened 

 on the outside of the book and the arrangement was exposed 

 to exceptionally bright sunshine from 11 A. M. to sunset, March 

 7. The plate when put into a developing bath behaved as if 

 unexposed. A prolonged development did not bring out a 

 trace of an image of the lead star. 



It will be remembered that Prof. Rontgen found that the 

 X-rays penetrated easily through a book of 1000 printed pages. 

 Indeed G. Moreau has recently stated that in his hands the 

 X-rays had penetrated through " several meters " of cardboard.* 

 So that the above experiment seems to be very significant. 



2. A piece of sheet aluminium l-2 mm thick was accurately 

 fitted into a frame. A very sensitive plate was placed behind 

 it and a lead star in front. With three hours' exposure not a 

 trace of an image conld be obtained. This experiment was 

 varied by substituting thin aluminium foil for the plate, also by 

 using bromide paper as the sensitive surface. No images in 

 any case were obtained. 



3. The sun's rays or some portion of its radiation passes 

 readily through wood, if the latter is not too thick. Thus 

 through a piece of white pine -f- & of an inch thick, images that 

 could readily be developed were obtained by three minutes 

 exposure to afternoon sunlight. With half an hour's exposure 

 the images were brilliant. 



A panel about 12 inches square was removed from an inside 

 shutter and replaced with a piece of white pine \ inch thick. 

 When the room was thoroughly darkened, reddish light could 

 be seen to pass through the board. So that wood of this thick- 

 ness is plainly translucent to the sight. 



The sun's light may be examined for X-rays also by fluores- 

 cence. 



*0. K, cxxii, p. 238: quoted Chem. News, Feb. 21, 1896, p. 85, (No. 1891.) 



