284 On the Intensification of Photographic Pictures. 



difficulty arises from the parallelism of the two surfaces of 

 the negative, and is obviated by using for the support of the 

 film a glass whose faces are inclined. The false light can 

 then be thrown to one side and rendered inoperative. In 

 practice it suffices to bring into contact with the negative 

 (taken as usual upon a parallel plate) a wedge-shaped glass of 

 equal or greater area, the reflexion from the adjoining faces 

 being almost destroyed by the interposition of a layer of 

 turpentine. By these devices the false light is practically 

 eliminated, and none reaches the sensitive film but what has 

 twice traversed the original negative. 



The other point requiring attention is to secure adequate 

 superposition of the negative and its image in the associated 

 reflector. On account of the slight lateral interval between 

 the copying-lens and the source of light, the incidence of the 

 rays upon the reflector is not accurately perpendicular, and 

 thus any imperfection of contact between the negative film 

 and the reflector leads to a displacement prejudicial to 

 definition. The linear displacement is evidently It sin 0, if t 

 denote the interval between the surfaces and the angle of 

 incidence, and it can be calculated in any particular case. It 

 is the necessity for a small t that imposes the use of a speculum 

 as a reflector. In practice 20 can easily be reduced to j* 2 ; 

 so that if t were ^ inch, the displacement would not exceed 

 ^q inch, and for most purposes might be disregarded*. 

 The obliquity could be got rid of altogether by introducing 

 the light with the aid of a parallel glass reflector placed at 

 45°; but this complication is hardly to be recommended. 



The scale of the apparatus depends, of course, upon the 

 size of the negatives to be copied. In my own experiments 

 ^-plates (4£ in. x 3£ in.) were employed. The condenser is of 

 plate-glass 6 in. diameter and 36 in. focus. The reflector is of 

 silver deposited on glass f. The wedge-shaped glass J attached 

 to the negative with turpentine is 4 x 4 ins. and the angle 

 between the faces is 2°. The photographic lens is of 3 inch 

 aperture and about 18 inch principal focus. It stands at about 

 36 inches from the negative to be copied. 



* If the glass of the negative were flat, its approximation to the 

 reflector might be much closer than is here supposed. 



t For a systematic use of the method a reflector of speculum metal 

 would probably be preferable. 



X It is one of thosi employed for a similar purpose in the projection 

 of Newton's rings (Proc. Roy. Inst. March, 1893 ; ' Nature,' vol. xlviii. 

 p. 212). 



