March 30, 1900.; 



SCIENCE. 



might arise from vapor given off by the 

 metal, copal varnish was used, but in place 

 of stopping the action it was found to in- 

 crease it, and this increase of activity was 

 traced to the turpentine contained in the 

 varnish. In experimenting with liquids it 

 is convenient to use small shallow circular 

 glass vessels such as are made for bacterio- 

 logical experiments, the plate resting on 

 top of the vessel, and the amount of liquid 

 in the vessel determining the distance 

 through which the action shall take place. 

 The following slide, produced in this way, 

 shows how dark a picture ordinary turpen- 

 tine produces. All the turpenes are active 

 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 phenom- 

 enon corresponding to what photographers 

 call reversal. The strong smelling bodies 

 known as essential oils, such as oil of berga- 

 mot, 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 Co- 

 logne, by cincamon, by coifee, 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 paraffine oil, 

 are without action on the x^hotographic 

 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 ma- 

 hogany. Again, the next slide you will 



recognize as the picture of a peacock's 

 feather. There is much interest in these 

 pictures of feathers, as they distinguish the 

 brilliant interference colors from those pro- 

 duced 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 bod}'^, and 

 it can, like the zinc and other active bodies, 

 act through considerable distances. In the 

 picture before yon 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 pic- 

 tures are in intensity. Printed pictures, of 

 course, act in the same way — here is a like- 

 ness 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 Royal 

 Institution ; also the rude trade- mark on 

 Wills's tobacco, and it is of interest because 

 the red pigment produces a very clear pic- 

 tm'e, 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 

 by sand-paper ; one is laid directly on the 

 photographic plate, and the other one has 

 a sheet of gelatin, its color 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 



