MEMOIR XV.] TAKING OF PORTRAITS BY PHOTOGRAPHY. 217 



Partly owing to the intensity of such light, which can- 

 not be endured without a distortion of the features, but 

 chiefly owing to the circumstance that the rays descend 

 at too great an angle, such pictures have the disadvan- 

 tage of not exhibiting the eyes with distinctness, the 

 shadow from the eyebrows and forehead encroaching on 

 them. 



To procure fine proofs, the best position is to have the 

 line joining the head of the sitter and the camera so ar- 

 ranged as to make an angle with the incident rays of less 

 than ten degrees, so that all the space beneath the eye- 

 brows shall be illuminated, and a slight shadow cast from 

 the nose. This involves obviously the use of reflecting 

 mirrors to direct the ray. A single mirror would answer, 

 and would economize time, but in practice it is often con- 

 venient to employ two; one placed, with a suitable mech- 

 anism, to direct the rays in vertical lines; and the second 

 above it, to direct them in an invariable course towards 

 the sitter. 



On a bright day, and with a sensitive plate, portraits 

 can be obtained in the course of five or seven minutes, in 

 the diffused daylight. The advantages, however, which 

 might be supposed to accrue from the features being 

 more composed, and of a more natural aspect, are more 

 than counterbalanced by the difficulty of retaining them 

 so long in one constant mode of expression. 



But in the reflected sunshine the eye cannot support 

 the effulgence of the rays. It is therefore absolutely 

 necessary to pass them through some blue medium, which 

 shall abstract from them their heat, and take away their 

 offensive brilliancy. I have used for this purpose blue 

 glass, and also ammonia-sulphate of copper, contained in 

 a large trough of plate glass, the interstice being about 

 an inch thick, and the fluid diluted to such a point as to 

 permit the eye to bear the light, and yet to intercept 



