280 Mr. W. Crookes on [Feb. 10, 



square pieces of pith at each end coated with lampblack. The whole is 

 fitted up and hermetically sealed in glass, and connected with an im- 

 proved mercury-pump. In front of the square of pith at one end a 

 quartz window is cemented to the apparatus ; and the movements of 

 the beam, when radiation falls on the pith, are observed by a reflected 

 ray of light on a millimetre-scale. The apparatus was fitted up in a 

 room specially devoted to it, and was protected on all sides, except where 

 the rays of light had to pass, with cotton-wool and large bottles of water. 

 A heliostat reflected in a constant direction a beam of sunlight, which was 

 received on an appropriate arrangement of slit, lenses, and prisms for pro- 

 jecting a pure spectrum. Results were obtained in the months of July, 

 August, and September ; and they are given in the paper graphically as 

 a curve, the maximum being in the ultra-red, and the minimum in the 

 ultra-violet. Taking the maximum at 100, the following are the mechanical 

 values of the different colours of the spectrum : — 



Ultra-red 100 



Extreme red 85 



Red 73 



Orange 66 



Yellow 57 



Green 41 



Blue 22 



Indigo 8| 



Violet 6 



Ultra-violet 5 



A comparison of these figures with those usually given in text-books 

 to represent the distribution of heat in the spectrum is a sufficient 

 proof that the mechanical action of radiation is as much a function of 

 the luminous rays as it is of the dark heat-rays. 



The author discusses the question, " Is the effect due to heat or to 

 light ?" There is no real difference between heat and light ; all we can 

 take account of is difference of wave-length ; and a ray of a definite re- 

 frangibility cannot be split up into two rays, one being heat and one light. 

 Take, for instance, a ray of definite refrangibility in the red. railing on 

 a thermometer it shows the action of heat, on a thermopile it produces 

 an electric current, to the eye it appears as light and colour, on a photo- 

 graphic plate it causes chemical action, and on the suspended pith it 

 causes motion. But all these actions are inseparable attributes of the 

 ray of that particular wave-length, and are not evidence of separate 

 identities. 



The author enters into some theoretical explanations of the action of 

 the different parts of the spectrum but these cannot well be given in 

 abstract. 



An experiment is described by which sunlight was filtered through alum, 



