Principles of Photography. 215 



from that having the longest focus being the product of the 

 longest focus and the distance between the lenses, divided by 

 the sum of their foci ; the distance between the lenses should 

 be about the tenth part of the sum of their focal distances. 

 There is no spherical aberration, if the right lines of the object 

 are undistorted in the picture, and the extreme portions are as 

 perfect as the central. 



Chromatic aberration or dispersion causes the borders of a 

 picture in the camera to exhibit prismatic colours; it arises from 

 the unequal refrangibility of the coloured rays, and prevents 

 the coincidence of the actinic and luminous foci. It is obviated 

 by such a combination of convex and concave lenses, as will 

 neutralize dispersion without altogether destroying conver- 

 gence. The greater the perfection of the acromatism, the 

 greater the difference between the optic and chemical foci. 



As the quantity of light is inversely proportional to the 

 square of the distance, it might be supposed that, the nearer 

 the object, the shorter the time required for exposure. Such, 

 however, is not the fact, since the longer the focus the more 

 perfect ; and hence, before the nearer objects in a landscape 

 are done, the horizon and other distant parts will be solarized. 

 This is not important in negatives, as the transparence is still 

 sufficiently perfect to secure an agreeable effect in the positive. 

 The Compounds of Silver, Gold, etc. Pure oxide of silver is 

 reduced by the prolonged action of light ; dissolved in am- 

 monia, it is far more sensitive. Perfectly pure nitrate of silver, 

 crystallized, or in solution, is not affected by light ; but when 

 moist, the presence of organic matter, such as starch, gelatine, 

 cellulose, etc., even in minute quantities, causes it to blacken 

 in the feeblest ray; hence the use of albumen on paper. 

 The organic matter is destroyed by nitric acid, oxide of silver 

 being liberated ; part of this oxide is decomposed by light ; the 

 remainder may be removed by ammonia, in which it is soluble. 

 The re-action of organic matter on nitrate of silver causes 

 paper imbued with it, unless absolutely free from moisture, to 

 become yellow in a few hours : more vigorous pictures are 

 obtained with paper in which partial reduction of the silver has 

 commenced. The nature of the organic body determines the 

 colour which results ; and the effect is heightened by pre- 

 cipitating the organic solution with a solution of lead, which, 

 under the influence of light, exalts the oxidation of the silver 

 salt, applying the precipitate to the paper in the consistence of 

 cream, and drying before the nitrate is applied. Chloride of 

 silver is very sensitive, and it is rendered more so by the 

 presence of a little nitrate. The colour it affords depends on 

 the chloride used to precipitate the nitrate ; in nearly all cases, 

 however, an olive tint is produced at last ; and, when light no 



