356 THE BEGINNINGS OF FBOTOGKAPHY. 



resinous and other organic substances shown l)y kSeneliier to bo sensi- 

 tive to light. He makes no mention of Sene])ier; but, had lie known 

 of his work, he would probabl}' have done so, as he recognized that all 

 bodies were more or less sensitive to light. 



Kitter must have been one of the tirst to recognize the electrical 

 nature of sunlight and the unit}^ of principle in the i)olarity of chem- 

 istry, electricity, magnetism, and heat. I can not now dwell upon this 

 aspect of his work, but in the light of modern physico-chemical science 

 it seems worth}^ of attention. 



DOt^TOR WOLLASTON. 



The existence of the ultra violet rays ygns also noticed by Dr. J. H. 

 Wollaston in 1801, about the same time as they were observed b}' 

 Ritter. In a note to a conununication to the Eoyal Societ}' (Phil. 

 Trans., 1S02, p. ;-i79), he says: 



Although what I have above described comprises the whole of the prismatic spec- 

 trum that can he rendered visible, there also pass on each side of it other rays 

 whereof the eye is not sensible. From Doctor Herschel's experiments (Phil. Trans., 

 1800) we learn that on one side there are invisible rays occasioning heat that are less 

 refrangil^le than red light, and on the other I have myself observed (and the same 

 remark has been made by ]\Ir. Ritter) that there are likewise invisible rays of another 

 kind tliat are more refracted than the violet. It is by their chemical effects alone 

 that the existence of these can be discovered, and l)y far the most delicate test of 

 their presence is the white muriate of silver. 



To Scheele, among many valuable discoveries, we are indebted for having first 

 duly distinguished between radiant heat and light (Traits de I'Air et du Feu, sees. 

 56, 57), and to him also we owe the observation that when muriate of silver is exposed 

 to the common prismatic spectrum it is blackened more in the violet than in any 

 other kind of light (sec. 66). In repeating this experiment I found that the black- 

 ness extended not only through the space occupied by the violet, but to an equal 

 degree and to al)Out an equal distance beyond the visible sj)ectrum, and that by 

 narrowing tlie pencil of light received on the prism the discoloration may be made 

 to fall almost entirely l)eyond the violet. 



In a sul)sequent communication to Nicholson's Journal (Vol. VIII, 

 1801:, p. l2l>3), he explains that in the above note he w^as careful to 

 express the power exerted ])y the most refrangible ra^'s on nuiriate of 

 silver in general terms as chemical, not merely f i-om a doubt whether 

 they would in other cases produce a corresponding etfect. but becaitse 

 he had at that time made experiments which proved that the same ra3^s 

 which cause the emission of oxygen by nuiriate of silvi^' occasion its 

 a])soi-ption \)y the resin usually called gum guaiacum. wliich turns 

 green by absorption of oxygen when exposed in the air to sunshine, 

 and consequently he ol)jected to Ritter's designation of the ultra violet 

 ra3\s as disoxidizing. 



He adopted an ingenious method of obtaining prismatic images for 

 the purpose of his experiments, and in this way must have been one of 

 the first to produce a photographic image on silver chloride by means 



