SCIENTIFIC NOTES. 145 



vacuum ; directly a light is brought near it in this condition, the 

 cross at once begins to revolve with a velocity in proportion to the 

 intensity of the light ; such a light as that given by burning 

 magnesium makes it spin till the discs look like a solid wheel. 



Mr. Crookes thinks they will make perfectly satisfactory 

 measures of the relative intensities of light submitted to them ; 

 and hence the name radiometer. Opinion is, of course, divided 

 about this startling instrument. Mr. Crookes, in reply to an 

 attempt made to explain the phenomena by the effect of the heat 

 rays in light, asserted publicly that " he was quite sure that by 

 selecting experiments from the vast number he had made, he could 

 prove or disprove any theory that might be brought forward to 

 explain the phenomena " ; and that before forming any theory of 

 the cause or mode of action, he would wait until the general testi- 

 mony of all experiments left no doubt about the true explanation. 



One of the experiments I saw may be mentioned. If a dark 

 source of heat — for instance, a bottle of boiling water — were placed 

 near the radiometer, the arms would not move even if a powerful 

 source of light were brought near them, but upon removing the 

 water bottle, and giving a few seconds to cool the side of the 

 radiometer, the arms at once started off again. 



Another very remarkable discovery was exhibited at the meet- 

 ing of the Royal Society, at which I exhibited our photos, and 

 drawings of the. transit of Venus, viz. : Mr. Spottiswoode's artificial 

 aurora. A vacuum tube about 20 inches long was connected with 

 a coil, and a beautiful stratified glow discharge filled it ; about the 

 centre of it, on the outside, was an electro-magnet, and directly 

 this was connected with the battery the rings of stratification took 

 on a peculiar rolling sort of screw motion which was very beautiful. 

 It is difficult to get the electrical forces nicely balanced, but the 

 effect while it lasted was very fine, and I need hardly say at once 

 suggested an explanation of the aurora, which it is probable exists 

 under similar conditions. I believe Mr. Spottiswoode is still in- 

 vestigating the matter. 



Passing from the science to the practical application of light, 

 there are two or three facts which may be of interest. Dry plate 

 processes of photography are very old, and have gradually acquired 

 a recognised place in the art, though second to the wet plates, with 

 which all the finest results have been obtained. Now, however, 

 Mr. Kennett, of Maddox-street, Regent-street, London, has dis- 

 covered a process which is better, quicker, and far less trouble to 

 the operator than the well-known wet-plate process. The sensitive 

 material, called by him pellicle, is sold dry and hard, something in 

 appearance like starch. It only requires dissolving in pure water 

 and putting on the plate like ordinary collodion. When dry it is 

 ready for use, and may be used then or six months afterwards. 

 The development also may take place after exposure or at conve- 



