142 Dr. William James Bussell [May 5, 



bodies. Dipentine is remarkably so ; in a very short time it gives a 

 black picture, and if the action be continued, the dark picture passes 

 away, and you then have a phenomenon corresponding to what photo- 

 graphers call reversal. The strong smelling bodies known as essen- 

 tial oils, such as oil of bergamot, oil of lavender, oil of peppermint, oil 

 of lemons, etc., are all active bodies, and all are known to contain in 

 varying quantities different terpenes ; therefore ordinary scents are 

 active bodies, and this is shown by the following pictures produced by 

 eau de Cologne, by cinnamon, by coffee, and by tea. Certain wines 

 also act in the same way : Sauterne gives a tolerably dark picture, but 

 brandy only a faint one. Other oils than these essential ones are also 

 active bodies : linseed oil is especially so ; olive oil is active, but not 

 nearly as much so as linseed oil ; and mineral oils, such as paraffin oil, 

 are without action on the photograjuiic plate. 



Interesting results are obtained with bodies which contain some 

 of these active substances ; for instance, wood will give its own picture, 

 as is shown by the following slides : the first is a section of a young 

 spruce tree, the next a piece of ordinary deal, and the third of an old 

 piece of mahogany. Again, the next slide you will recognise as the 

 picture of a peacock's feather. There is much interest in these pictures 

 of feathers, as they distinguish the brilliant interference colours from 

 those produced by certain pigments ; the beautiful blue in the eye of 

 the peacock's feather is without action on the photographic plate. 

 Butterflies' wings, at least some of them, will draw, as you see, their 

 own pictures. Linseed oil, which is a constituent of all printing ink, 

 makes it an active body, and it can, like the zinc and other active 

 bodies, act through considerable distances. In the picture before 

 you the ink was at a distance of one inch from the plate, and the 

 next slide shows what a remarkably clear and dark picture ordinary 

 printing can produce. As the composition of printing ink varies 

 so does its activity, and here are pieces of three different newspapers 

 which have acted under the same conditions on the same plate, and 

 you see how different the pictures are in intensity. Printed pictures, 

 of course, act in the same way — here is a likeness of Sir H. Tate 

 taken from " The Year's Art." The pictures and printing in Punch 

 always print well ; so does the yellow ticket for the Friday evening 

 lectures at the Eoyal Institution ; also the rude trade-mark on Wills's 

 tobacco, and it is of interest because the red pigment produces a very 

 clear picture, but the blue printing is without action on the plate. 



An interesting and important peculiarity of all these actions is 

 that it is able to pass through certain media ; for instance, through a 

 thin sheet of gelatin. Here are two plates of zinc; both have been 

 scratched 1 iy sand-paper ; one is laid directly on the photographic 

 plate, and the other one has a sheet of gelatin, its colour is of no 

 note, laid between it and the sensitive plate ; the picture in this case 

 is, of course, not so sharp as when no gelatin is present, but it is a 

 good and clear likeness of the scratches. 



Celluloid is also a body which allows the action to pass through 



