THE BEGINNINGS OF PHOTOGRAPHY. 357 



of a len.s. Over a lens seven inches in diameter and about twenty- 

 four and one-fourth inches focus he pasted a circular piece of paper 

 liaving its radius one-tenth of an inch less than that of the lens, 

 thus leaving" a prismatic annulus corresponding in the length of its 

 circumference to a prism twenty-two inches long, so arranged by 

 its circular form that anj^ one of the colors might at pleasure 

 l)e Ijrought to a focus, or the spectrum could be received on a ring 

 of any diameter required by mere variation of the distance of the 

 lens. At short distances the exterior margin of the spectrum was 

 red and the violet within; at greater distances than the focus the order 

 of the colors was reversed, the violet being on the margin and the red 

 within. With this apparatus he found that the etfect on nuiriate of 

 silver was much accelerated. At distances short of twenty-two and 

 one-fourth inches a ring was produced, at twenty-two and one-half a 

 circular dark- colored spot, and at about twenty-three inches appeared 

 to be the focus of these rays, as the spot was then smallest; at twent}"- 

 three and one-half it was larger, at twenty-four and one-fourth it again 

 became a ring shaded to the center, and at twenty- four and one-half 

 (unless the paper had been wetted) the center remained completely 

 white, though strongly illuminated. He was, however, unable to 

 restore the white color to the muriate after it had once been tinged, 

 however slightly, by exposure to the most refrangible rays. Similar 

 experiments were tried with the gum guaiacum on paper tinged with 

 an alcoholic solution of the gum. 



THOMAS AVKDGWOOD. 



We now come to Thomas Wedgwood's experiments. Though they 

 may have been carried out some A'ears l)efore they were published, I 

 have thought it better to discuss them in order of the date of their 

 publication hy Davy in 1802. Miss Meteyard mentions (Life of 

 Wedgwood, 2, 586) with regard to Thomas Wedgwood's early work: 

 "His father early in 1774 had used the camera obscura for taking 

 views for the Russian service, and Doctor (Matthew) Turner, of 

 Liverpool, as it was well known, had either invented or brought to 

 toleral)le pei-fection the art of copying prints upon glass by striking 

 oflf impressions with a colored solution of silver and tixing them on 

 the glass by baking on an iron plate in a heat siifficient to incorporate 

 the solution with the glass. 'SMth knowledge thus ()))tained and 

 o])Scrvation directed, it amounts to absolute certainty that Thomas 

 Wedgwood, during some of his experiments on the production of light 

 from different bodies by heat and attrition, made certain discoveries 

 which led practically to the first principles of photograph}'. '' She 

 goes on to discuss his subsequent experiments with Davy and the 

 difficulty of tixing the images, and falls into the mistake about the two 



