Principles of Photography. 213 



tint. Yellow and red are, to the photographer, dark colours ; 

 blue and rose, bright ones. The green of leaves is inert with 

 regard to compounds based on iodide of silver, but energeti- 

 cally affects those based on bromide of silver ; hence, one of the 

 advantages of using both iodine and bromine in the preparation 

 of collodion. The sensibility of the eye to the solar rays, is 

 greatly less than that of the substances used in photography : 

 during the total eclipse of July 18, 1860, a photographic image 

 of prominences, not visible with glasses, was obtained. The 

 rays at the red extremity of the spectrum are too slow, and 

 those at the violet extremity too rapid, to affect the eye ; just 

 as sounds may be too high or too low to be audible. Grlass 

 will not transmit freely the more refrangible actinic rays ; the 

 only solids which allow them a free passage are diamond, rock- 

 crystal and fiuor spar ; water is the only fluid ; hydrogen, 

 nitrogen, oxygen, and carbonic acid the only gases. The 

 atmosphere transmits varying amounts of actinic rays : from 

 noon until six in the afternoon, in summer, about twice as long 

 exposure is required as between nine in the morning and noon. 

 Light produces a more or less permanent effect on bodies ; if an 

 engraving is exposed to the sun, and placed on a sheet of 

 sensitized paper, it will reproduce a trace of the object on the 

 paper. The chemical action of light is modified by the greater 

 or less intensity of what is reflected from the various parts of 

 an object, according as they are more or less in shade, or more 

 or less averted ; and hence lights and shades are reproduced 

 with their proper gradations. To secure rapidity, it is necessary 

 to condense the light ; and hence, in ordinary cases, the image 

 is less than the object. To obtain a good picture, we must 

 have, not only a good apparatus, but a good light ; those who 

 pretend to produce as good results on dull as on bright days, 

 deceive themselves, or attempt to deceive others. 



Although no amount of excellence in the instrument will 

 make up for insufficiency of light, its quality is of great 

 importance. When Daguerre made his experiments, an 

 exposure of a quarter of an hour at least was required ; when 

 the camera was improved, three minutes sufficed in similar 

 circumstances. Short foci, and two lenses, which intensify the 

 light, and thus give rapidity, are required for portraits ; for 

 landscapes, where the objects are in different planes, single 

 objectives with long foci and diagrams or stops, are employed; 

 although these increase the time required for exposure. If a 

 portrait objective is used with a landscape, a diaphragm with a 

 very small opening will be required ; this enlarges the field, 

 but, at the same time, enormously diminishes the rapidity. 

 With a single lens, a larger number of distant objects are in 

 focus j but even this is improved by a diaphragm, which ex- 



