MfcTHOD OP COPYING PAINTINGS UPON GLAS*. Jfj? 



VI. 



An Account of a Method of copying Paintings upon Glafs, and of 



making Profiles by the Agency of Light upon Nitrate of Silver. 



. Invented by T. Wedgwood, Efq. With Obfervations by 



H. Davy. From the Journals of the Royal Injlitution, 



I. 170. 



W HITE paper, or white leather, moiftened with folution White paper, 



of nitrate of filver, undergoes no change when kept in a dark * c * we " ed Wlt *» 



i • - . r , , i i- i • r im n't J ate of filver, 



place; but, on being expofed to the day-light it fpeedily becomes black by 



changes colour, and, after paffing through different (hades of ll S> ilti 



grey and brown, becomes at length nearly black. 



The alterations of colour take place more fpeedily in pro- with different 



portion as the light is more intenfe. In the dired beams of the !" p " d as the 1! S ht 

 r o is ftronger j e. g» 



fun, two or three minutes are fufficient to produce the full a minutes in 

 effect. In the fhade feveral hours are required, and light tranf- funfl »' ne « 

 mitted through different coloured glaffes, acts upon it with 

 different degrees of intenfity. Thus it is found that red rays, Coloured rays 

 or the common fun-beams paffed through red glafs, have very a ^ ^ ,th g? er " 

 little a6lion upon it : yellow and green are more efficacious ; 

 but blue and violet light produce the mofl decided and power- 

 ful effeas*. 



The 



* The fa£ls above mentioned are analogous to thofe obferved long 

 ago by Scheele, and confirmed by Senebier. Scheele found, that in 

 the prifmatic fpe&rum, the effeel produced by the red rays upon mu- 

 riate of filver was very faint, and fcarcely to be perceived j whilft it 

 was fpeedily blackened by the violet rays. Senebier ftates, that 

 the time required to darken muriate of filver by the red rays, is 20 

 minutes, by the orange 12, by the yellow 5 minutes and 30 feconds, 

 by the green 37 feconds, by the blue 29 feconds, and by the violet 

 only 15 feconds Senebier fur la Lumiere, Vol. III. p. 199. 



Some new experiments have been lately made in relation to this 

 fubjecl:, in confequence of the difcoveries of Dr. Herlchel concern- 

 ing the invifible heat-making rays exifting in the folar beams, by 

 Meffrs. Ritter and Bockmann in Germany, and Dr. Wollafton in 

 England. 



It has been afcertained by experiments upon the prifmatic fpec- 

 trum, that no effects are produced upon the muriate of filver by the 



invifible 



