84 Lord Rayleigh on the Manufacture and 



necessary. In an early stage of an investigation, when the 

 causes of failure are numerous and unknown, it is best to exclude 

 every thing that can possibly be supposed to be prejudicial. 

 When the principal difficulties have been overcome, it will be 

 time enough to determine what precautions are necessary, if the 

 question has not been already settled by accidental experience. 



In the case of developed plates there is more choice of lights 

 in consequence of the higher sensitiveness. I have used success- 

 fully cloud or skylight reflected horizontally from the zenith by 

 a mirror through a hole of two or three inches diameter in the 

 shutter of a darkened room, the frame being set up in a vertical 

 plane at a few feet distance. The principal objection to this 

 plan is the difficulty of estimating the exposure with proper pre- 

 cision — a difficulty which is more felt than in ordinary photo- 

 graphy, as it is convenient to develop a good many copies at 

 once. On a really fine day the image of the sun formed by a 

 condensing-lens of short focus placed in the shutter (as used in 

 diffraction experiments) constitutes a very convenient source of 

 light. As the exposure is only a few seconds, there is no diffi- 

 culty in dodging isolated clouds, whose progress may be 

 watched from within by examining their image with a coloured 

 glass. When there is any haze, this method is not more satis- 

 factory than the other. 



With the more sensitive processes artificial light may be em- 

 ployed. I have done a good many copies by the aid of a mode- 

 rator-lamp (without the globe) at two feet distance from the 

 frame. An Argand gas-flame would probably be still better. 



The printing-frame I employ has a thick plate-glass front, 

 against which the original grating and the prepared plate are 

 pressed by screws. These are more under control than the 

 springs generally used in the common printing-frames. When 

 every thing is ready, the original is placed on the glass front of 

 the frame with the engraved face upwards, care being taken to 

 exclude all grit by means of a camel's-hair brush. The prepared 

 plate is then placed face downwards on the grating, then a pad 

 to equalize the pressure (I have used one of india-rubber), and on 

 the pad the rigid back of the frame, on which the screw r s are 

 made to press with a moderate force. When the film is delicate, 

 care should be taken to place it in the proper position at once 

 without sliding. 



The two surfaces of the plate-glass front of the printing- frame 

 and the back of the original grating may be cleaned in the ordi- 

 nary way with a soft cloth or wash-leather; but the engraved 

 face of the grating requires more delicate treatment. If touched 

 at all with a solid (wash-leather), the greatest care should be 

 used. I prefer to wash it, when soiled, with a stream of water 



