Eevei'sals hi S^yectram Photographs. 



579 



to >omo others. Reversals of this ty])e Avill be seen in fig. G 

 illiistratino- my paper on screens transparent to nltra-violet 

 li<ilit*c The s})ectra (negative) have bright borders. 



Fourth Tiipe. — The C'hiyden effect, which is the type chiefly 

 to be dealt With in the present paper. This occurs when an 

 exposure of about 1/1000 of a second or less is given, and 

 the ;'late subsequently fogged by exposure to diffuse light 

 before develoj)nient. If images of electric s})arks are thrown 

 on a plate and the plate then exposed to the light of a candle 

 for a few seconds, the spark images will develop reversed, 

 which is not the case if the exposure to candle-light precedes 

 the impression of the spark images. 



Tiiis effect is shown in fig. A. A series of spark images 

 have been gradually reversed by exposing the plate in strips 



Flo, A. 



to the light of a candle. The upper strip was exposed 

 30 seconds, the following 20, 15, 10, 5, 2, respectively. 



Fifth Type. — I have found that the condition produced in 

 the sensitive film by light shock can be imitated by treating 

 the plate, after exposing portions of it to the action of a feeble 

 light for a few seconds, to an oxidizing bath of bichromate 

 of potash and nitric acid. If the plate is dried and then 

 fogged by candle-light and developed, the previously exposed 

 portions will come out reversed. 



This effect is shown in Plate XXVI. fig. 4. A series of spark 

 images was impressed on the plate, which was then covered 

 with a piece of black paper in which a narrow slit had been 

 cut. A number of images of this slit were then impressed 

 on the plate by exposure to the light of a candle. If the 

 plate in this condition w^ere then fogged by candle-light and 



* Phil. M(io-. Februarv 11)03. 



