Stereoscopic Photographs. 151 



may be moved into its second position by pushing along the 

 guides, without the trouble of a second arrangement of the 

 composition. With a camera of half-plate size, or larger, the 

 same result may be obtained by having a sUding front which will 

 permit the lateral movement of the lens for the required distance, 

 the interior of the camera being divided by an opaque partition, 

 so as to prevent the light striking both sides of the plate at once. 



These systems of course are only apphcable to still-life subjects; 

 where movement is likely, or when instantaneous effects are 

 desired, twin lenses are essential. The lenses are mounted side 

 by side at a distance of about 2^ to 2f in. apart, and the two 

 pictures exposed simultaneously by hand or shutter, as may be 

 convenient. 



The most suitable lenses to employ are single landscape lenses 

 of from 4 J to 6 inches focus ; as although, owing to spherical 

 aberration, the image of straight lines is sometimes distorted by 

 these instruments, this distortion is corrected by the stereoscope, 

 and true rectilinearity obtained. Any lenses are suitable, how- 

 ever, provided their focal length does not exceed six inches ; but 

 it should be remembered that an absolutely rectilinear photograph 

 may be distorted by the prismatic lenses of the stereoscope. For 

 those who are purchasing new lenses for stereoscopic work, 

 single landscape lenses offer every advantage, as they are 

 considerably cheaper than rectihnear lenses of the same class. 



The exposure, especially if twin lenses are employed, is most 

 conveniently made by means of a shutter, which will give a shorter 

 or longer exposure at will. The drop-and-flap pattern is very 

 suitable for this work, and has been used for some time by me 

 with success. It is not my intention to give directions for deve- 

 loping the negative. I presume everyone to be acquainted with 

 ordinary photographic manipulation ; but it may be necessary 

 to say that a very full exposure should be given, so as to get a 

 soft, thin negative, full of detaU ; under-exposure, dense, or hard 

 negatives should be avoided, as the resulting picture will suffer 

 from what is known as the " snowy " appearance so common in 

 stereographs. 



Paper prints may be made by any convenient process — silver, 

 gelatino-bromide, or platinotype. If the high lights are chalky, 

 it is a good plan to "sun them down," that is, by exposure to 

 light to tint or degrade the highest lights. As the right-hand 

 side of the negative comes out on the left-hand side of the print, 

 it is necessary to reverse these in mounting on card, so as to 

 preserve a true stereoscopic effect, and avoid a pseudoscopic or 

 hollowed-out appearance. 



The success of the stereograph depends so much on accurate 

 mounting, that a few remarks on that point may be useful : — 



(1.) As soon as the print is ready, a straight cut should be 



