WEOATIVE PICTURES.] UNDULATORY FORCES. LIGHT PHOTOGRAPHY. 



151 



decided proofs of superiority. Generally speaking, each 

 maker maintains the regularity of the article lie sells ; 

 and when once the student has so far succeeded in 

 adapting his bath and developing solutions to one kind 

 of collodion, he will only have the trouble of going over 

 the old ground again, in attempting to suit his practice to 

 the production "of a new maker. Much, therefore, 

 depends on making a first and proper choice of the 

 collodion. 



For the silver bath, a thousand-and-one advices and 

 recipes have been given : indeed, their name is legion. 

 The object of course is, that when the collodionised plate 

 is immersed in the bath, its surface shall be made as 

 highly sensitive as possible, and that each part should be 

 equally so when compared with another. Sometimes the 

 bath is too acid, and at others too alkaline. The former 

 evil is remedied by the addition of a small piece of 

 kaolin ; and the latter by the addition of a drop or two 

 of nitric or acetic acid. Now, without going far into 

 practical chemistry, we may state, that a piece of litmus 

 paper will turn red in an acid ; and a piece of turmeric 

 paper turns to a deep brown in an alkaline solution. 

 These may easily be obtained ; and by them the bath may 

 at once be tested. The strength of the bath is subject to 

 continual variations ; because, every time a collodionised 

 plate is introduced, a portion of silver is withdrawn, and 

 a small portion of iodine is left behind. Now these facts 

 must be borne in mind, because they necessarily influence 

 the process to a great extent. Fogging is the result of 

 an improper state of the bath ; and from whatever cause 

 such may arise, it is of importance that the remedy should 

 be instantly applied. This tendency is generally removed 

 by the addition of a small portion of nitric acid, so that 

 the super-alkalinity of the bath may be destroyed, and a 

 very slight acidity be produced. This is a point which, 

 having named, we must leave to the judgment and skill 

 of the operator. 



Besides the alterations of the bath, produced by the 

 causes to which we have referred, the student must 

 bear in mind, that each time a plate is introduced, a 

 small quantity of ether and alcohol is left behind. This 

 produces a decided harmful effect in the end ; and the 

 only plan is to renew the bath entirely. 



We may here state, that the silver can oasily be 

 recovered, in the form of chloride, from used-up baths, 

 by precipitating with a solution of common salt. It 

 may be reduced, by means of heat in a crucible, to the 

 metallic state, if mixed with cyanide of potassium ; 

 which, however, is perhaps too troublesome a process 

 to most of our readers. Photographic chemists and 

 metallurgists, however, will purchase it at its commer- 

 cial value ; and this hint may therefore be of use to 

 those whose means do not permit of extravagance in 

 their photographic pursuits. 



The developing solutions, like the sensitising bath, 

 have been the subject of enormous variation in the 

 proportion of their ingredients. We shall not enlarge 

 thereon, because we shall only fall into the error of 

 those who have preceded us, by giving advice for 

 circumstances over which we can have no control, not 

 being present to observe them. The student should 

 watch the effect of the different strengths of the 

 solutions he employs, and he will thus gain more 

 knowledge by a few trials, than he could do by perusing 

 any number of works on the subject. 



In using the fixing solution of the cyanide of potas- 

 sium, care should be taken that it is of not too great 

 strength : it will be therefore advisable to use a weak 

 one', and incur the expenditure of a little extra time, 

 rather than to spoil the picture by employing one of 

 greater strength for the purpose of speed. The motto 

 of "hasten slowly," is, generally speaking, a piece of 

 sound philosophy in the practice of photography. 



Abundance of washing removes almost every chance 

 of subsequent change in a positive picture. We strongly 

 advise that the last washings should always be per- 

 formed by means of distilled water. 



In choosing the transparent varnish for the collodion 

 surface, care should be taken that it is such as not to 



crack when dry. The same may be said of the black 

 varnish used for backiug-up the picture. Each of these 

 can, generally speaking, be depended on when pur- 

 chased of the photographic chemists, as they specially 

 prepare these varnishes for the use of photographers. 



Having thus detailed the processes and the difficulties, 

 so far as obtaining positive collodion pictures is con- 

 cerned, we pass on to give directions for obtaining 

 negatives ; and shall then describe the best modes of 

 printing paper positives from them. 



NEGATIVE PICTURES BY THE COLLODION 

 PROCESS. 



WB shall now describe the methods which must be 

 adopted to obtain negative pictures, or those in which 

 the lights and shadows are the reverse of what we 

 find in nature. The chief utility found in these pic- 

 tures is, that they enable us to obtain any number of 

 positive copies from them ; and so, to a large extent, 

 they increase the value of photographic processes, and 

 approximate their application to a similar purpose of 

 reproduction, as that obtained by the ordinary process 

 of engraving. 



The remarks, advice, and precaution which we have 

 already afforded, are equally applicable in reference to 

 this part of our subject. We, however, shall find, that 

 in some respects, the materials employed, and the mani- 

 pulative processes, vary from those which are followed 

 in producing positive pictures ; and it will now be our 

 duty to point such out to our readers. 



The plates are to be prepared with scrupulous care 

 after the manner we have already stated, and are then 

 to be coated with collodion prepared expressly for the 

 purpose. This is generally sold under the name of 

 " negative collodion," and contains more pure collodion, 

 in each ounce of the liquid, tlian that employed for 

 positive pictures. The following recipe for negative 

 collodion is recommended by Mr. Hardwich : 

 Purified ether, sp. gr. 720 .... 6 drachms. 

 alcohol, sp. gr. "825 ... 3 



Soluble pyroxyline 3 to 6 grains. 



,, iodide of ammonium ... 3 to 4 ., 



Although we have introduced the above recipe, we 

 still advise the purchase of the collodion by the student, 

 as being the best course he can pursue. 



After having obtained a good coating on the surface 

 of the glass plate, it must be made sensitive by means 

 of the silver bath ; and in this, considerable variety of 

 constitution has been recommended. The student must 

 bear in mind, that a positive picture is required to give 

 a distinct appearance of surface by light reflected to the 

 eye ; and this requires but a moderately thin coating of 

 collodion. Now a negative picture must have depth 

 and body, because its value and effects depend, in use, 

 on the differences of tone produced by the light fraiw 

 mitted through it when paper copies are taken from it. 

 Hence the various modifications required to produce 

 these result* in negative pictures during the prepara- 

 tion of the plates. 



We annex some recipes for making the silver solution, 

 or bath, which we have gathered from various sources ; 

 and the student will do well first to try them in the 

 proportions named, which, however, he can modify 

 as his experience permits, and circumstances require. 

 We need scarcely repeat, that the ever-varying qualities 

 of the collodion he may employ will frequently require 

 such modifications in practice. A knowledge of this 

 can only be gained by experiment under special circum- 

 stances ; hence our duty cannot go further than to indi- 

 cate, as complete direction to ensure constant success is 

 entirely impossible. 



Silver Bath for Negative Pictures. 



Nitrate of silver . . . .30 grains. 



Iodide of potassium i grain. 



Distilled water .... 1 ounce. 

 This solution may be made by first dissolving the nitrate 

 in a portion of the distilled water, and the iodide in the 



