140 



UNDULATORY FORCES. LIGHT PHOTOGRAPHY. 



constructing a Urge magnetic machine, which U >lnvt<n by 

 team-power. Coils of wire, enclosing bars of toft iron, 

 are opposed to powerful magnet*. These rotate rapidly, 

 and, each time of passing the coils, a current of elect 

 U produced. In 1856, we were present at a trial of its 

 powers, at the promises of the Trinity Board, Blackwall ; 

 and we can, perhaps, give no better illustration of the 

 effect of this light, than by stating, that we could easily 

 detect the divisions or panes of a window in a house a 



distant, when the light was concentrated by mean 

 of the ordinary lighthouse lenses. The apparatus was 

 subsequently transferred to the South Foreland ; <uu 

 nightly, for six months, without a moment's interruption 

 the light was maintained, casting its brilliancy across tli 

 Channel, and being distinctly seen on the French coast. 

 \\ o shall reserve our further remarks on the electri 

 light fur the section of Electricity, under which it wil 

 more conveniently be discussed. 



CHAPTER IV 

 PHOTOGRAPHY. 



HAVIXO, in our former pages, entered fully into the ex 

 animation of the phenomena and laws of light, we are 

 prepared to deal with one of the most interesting appli- 

 cations of physical and chemical science namely, the an 

 of taking pictures by the agency of light. We shal 

 have to introduce the subject by referring to gome 

 of the laws and facts of chemistry; because, although 

 photography is an application of the effects of ligln 

 an an agent, the process is an entirely chemical one so 

 far as the operator is concerned. 



The power of light in changing the ordinary colour oi 

 objects, is far more universal than is generally supposed. 

 Most of out readers must have noticed that some delicate 

 colours speedily fade when exposed to the full light ol 

 the sun. This effect is undoubtedly duo to some action 

 produced by the rays of light on the substances with 

 which a material has been dyed. Light-blue and pink 

 si;ks> are thus easily faded. Sir John Herschel, in some 

 masterly researches, published a few years ago in the 

 Philosophical Transactions of the Royal Society, pointed 

 o;it, that all the juices of any flower are thus acted on 

 by light. Indeed, we may state generally, that the 

 cause of the varied colours of our landscapes and flowers, 

 uply that of a process analogous to those employed 

 in photography. One of the simplest illustrations of 

 the kind we can offer, is that of the different appear. 

 of the inside and outside of some vegetables, such as 

 celery, lettuce, ami tho cabbage. The external leaves 

 whieh have been freely exposed to the direct solar rays, 

 have a deep green, or, in celery, a red colour ; whilst the 

 inner leaves, which have been protected from light, 

 always present a pale or blanched appearance. Even 

 the human face may be taken as another instance of the 

 same kind ; for we observe the ruddy glow and deep 

 colour in the rustic, presenting a bright contrast to the 

 sallow, pale, and unhealthy appearance of the factory 

 operative, or the resident of our large cities. Light, 

 indeed, has a colourising and a sanitary effect, to an 

 extent little dreamt of by those unacquainted with the 

 science of physiology. 



Photographers, however, avail themselves almost en- 

 tirely of the metal silver as the great medium of their 

 operations. When that well-known metal is united with 

 certain substances, the resulting compounds are rapidly 

 nile-ted by light ; anil on exposure thereto, become 

 changed to a dark colour. The whole art of photo- 

 graphy may be almost summed up as existing in the 

 choice of the salts of silver, their proper application, and 

 imniedi:it>: removal when the desired effect is gained. 

 It is true that many other substances may be similarly 

 employed ; Imt, iu practice, they are of comparatively 

 little independent worth. 



It will be impossible to go into the entire history of 

 this most int. Test ing art, for its inventors and im- 

 provers are indeed "legion." Suffice it to say, that wo 

 are indebted to Mr. Wedgwood and Sir Humphry 

 Davy for the initiative of our processes ; and, since their 

 day, MM. Niepce and Daguerre, in France, and Messrs. 

 Tall)ot, Arc!i'-r, Hunt, Sir John Herschel, and a host of 

 other eminent men, have, by their intelligence and 

 practical skill, raised to the highest rank as a philoso- 

 phical study, and as a social and artistic occupation, 



that which, twenty years ago, was but barely named in 

 ordinary scientific works. 



It will be our endeavour to discuss and illustrate the 

 principles and practice of photography iu as plain : 

 manner as possible. For the sake of saving space, we 

 shall assume that our readers, through our previous 

 pages, have become acquainted with the various o[ 

 laws, instruments, <tc., to which we liave alluded. This 

 will prevent tedious and unnecessary repetition. Tim 

 chemical processes we shall describe as if we addressed 

 those who are unacquainted with chemical science ; and 

 whilst giving a variety of details as to processes em- 

 ployed or recommended by different individuals, we 

 shall mostly keep in view those which are simple, 

 effective, and easy of prosecution by persons of ordinary 

 intelligence, and limited scientific attainments. 



INTRODUCTORY EXPERIMENTS. 







WE commence by introducing the experimenter to a few 

 illustrations, which will give a general idea of the nature 

 of photographic processes, and which ho will do weli to 

 repeat until a certain amount of ease in manipulation is 

 gained. We must, however, preface our remarks by 

 urging on all a great point, without attention to 

 which all attempts at photographing must surely fail : 

 we refer to scrupulous cleanliness. Neglect of this, and 

 carelessness, are the two chief causes of all the dis- 

 appointments both of the tyro and practised operator ; 

 and to impress the necessity of our advice being attended 

 to, wo shall suggest an experiment, by which the cleanest 

 hands may be proved to be photograplu'cally dirty on 

 many occasions. 



Experiment 1. After washing and drying the hands in 

 the usual manner, allow them to remain some time in 

 distilled water, afterwards frequently rinsing them 

 therein. Add to the water a solution of nitrate of silver: 

 in a short time the water will assume a milky appear- 

 ance, owing to the common salt exuded with the perspi- 

 ration from the skin, forming a substance with the 

 silver, called chloride of silver. If a little of the solntu >n 

 of nitrate of silver be added to pure water, no salt of 

 the kind we have named will be produced. 



It will thus be seen how, in every case, the photo- 

 grapher carries with him the instruments of his own 

 destruction. Whenever a piece of paper or gla 

 touched by the lingers, then a spot of common salt, 

 grease, and organic matter is left : these acting on the 

 silver salts, produce patches and spots wliieh .spoil any 

 photograph. Wo trust that our readers will be by this, 

 fully persuaded of the necessity of care in this res; 

 nid thus they may prevent the numerous annoyances to 



liieh their neglect would otherwise expose them. 

 The salt formed during Experiment 1, is called tho 

 chloride of silver, lieeause the chlorine, in tho common 

 salt from tho hand, lias joined with tho silver contained 

 n the nitrate of that metal. We must here state, that 

 the term "salt" is applied by tho chemist to a v.ist 

 range of substances; and gem-rally, whenever an acid is 

 inited with an earth or an alkali, a "salt" is i'. Mined. 

 I'hus nitre, or saltpetre, is composed of nitric acid and 

 jotass, and is called a salt of potass. Its name is parti/ 



