PHOTOGRAPHY 



149 



merit* of a given case : for taking subjects in a 

 confined situation, such as a narrow street, or for 

 depicting the facade of a large building at very 

 limited range, a 'wide-angle' lens is necessary. 

 For tbe convenience of those cai Tying a number of 

 lenses, Ross lias devised a series of symmetrical ' 

 lenses that all tit one llange. Every lens is sold 

 with a set of diaphragms or stops ; and, in the case of 

 the lens last mentioned, these usually take the form 

 of a rotating plate, pierced with the necessary aper- 

 lures. lint in other lenses the diaphragms are in the 

 form of separate plates, and are kept in a small 

 leather case by themselves, the operator removing 

 any of the series, and inserting it in tbe lens when 

 wanted. These diaphragms are marked with what 

 is known as their focal value, which means that the 

 number marked represents the fraction which the 

 aperture of the diaphragm is of the focal length of the 

 lens. To make this plain, let us suppose that the 

 focal length of the lens is 6 inches, and that the dia- 

 p'.iragm in question has an aperture of a quarter of 

 a'i inrh. As there are in 6 inches 24 of such quarter 

 inches, the figure 24 will represent its value, and 



this will l>e expressed thus : ^. In like manner, a 

 half-inch diaphragm would be marked j~, and a 



one-eighth inch diaphragm ^. There is, however, 

 another method of marking lens stops, which hag 

 lieen adopted by the Photographic Society of 

 Great Britain, and which is called the ' filiform 



Standard, ' or U.S. for short. In this system - is 

 called No 1, and the U.S. number for any other 

 sixe is found by dividing the focal length of the 

 lens by the diameter of the o|iening as already 

 described, squaring the result, and then dividing 

 by 16. Of late years a remarkable improvement, 

 which, however, hail fur some time l>een applied to 

 microscopes, has been made in photographic lens 

 diaphragms, in a contrivance which, from its resem- 

 blaiire to the natural diaphragm of the eye, which 

 expands or contracts automatically according to the 

 amount of light to which the organ is subjected, is 

 called the iris diaphragm. This consists of a 

 iiumlK.-r of Hat blades or tongues of thin blackened 

 metal, which are fastened to a ring in the lens 

 mount. I!y a turn of this ring the blades expand 

 or contract, so that the aperture can l>e enlarged or 

 diminished as required, while a scale marked on the 

 outside of the lens mount, and a travelling printer, 

 indicate the focal value of every change of aperture. 

 We may mention here one more important change 

 in lens manufacture the introduction of alu- 

 minium instead of brass for the necessary metal 

 work, by which the weight of the instrument is 

 greatly reduced. The same useful metal, only now 

 rendered available by its cheaper manufacture, is 

 also coming into use as a. substitute for brass in the 

 other metal fittings of cameras. A new kind of 

 known as Jena optical glass, is now lieing 

 employed in the construction of photographic lenses. 

 By its use a larger field can l>e covered with a 

 given aperture than was the case under former 

 conditions. 



Wet-collodion Process. Collodion (q.v.) is the 

 name given to the solution of pyroxyline, a kind of 

 gun cotton, in a mixture of 'ether and alcohol. 

 When I his is (lowed over a glass plate it gradually 

 dries into a transparent film. It was first introduced 

 for photographic purposes in IK.")] by Mr Scott 

 Archer, and has been of great and important wrvice. 

 For fully a quarter of a century the wet-collodion 

 - was almost exclusively practised by photo- 

 graphers in the earlier years for the production of 

 positives on glass, and occasionally on leather or 

 other non-fragile materials; latterly by modifications 

 the process was more extensively employed for the 



production of negatives. Dry-collodion processes 

 have also been in use, although on a much more 

 limited scale. These are the stages in the wet-collod- 

 ion process : ( 1 ) A glass plate made perfectly clean 

 is coated with collodion, to which the bromide of cad 

 mium and either iodide of potassium or iodide of 

 ammonium have been added. (2) The collodion- 

 Lsed plate is 'sensitised' by immersion in a bath 

 of nitrate of silver, containing 35 grains to every 

 ounce of distilled water. (3) Production of latent 

 image by exposing the sensitised plate in the camera 

 after the object has been focused. (4) Development 

 of latent into visible image by flooding the plate 

 with a solution of sulphate of iron (ferrous sul- 

 phate), or of pyrogallic acid, to either of which 

 some acetic or citric acid is added. (5) Fixing of 

 the permanent image by immersion of the plate in 

 some solvent of those parts of the sensitive surface 

 upon which the light lias not acted. This solvent 

 for wet plates is cyanide of potassium, but for more 

 modern processes hyposulphite of soda is employed. 



Dry-mute Processes. It is hardly necessary to do 

 more tlian name a few of the earlier dry-plate pro- 

 cesses, since, except for experimental purposes, they 

 have all been beaten out of the field by the recent 

 one known as the gelatinp-bromide process. Several 

 advantages arise, especially for field-work, from 

 using dry sensitive plates. With the wet process 

 the operator, when away from his studio, must 

 take with him a dark tent, collodion, a silver bath, 

 and developing agents, besides a supply of water 

 for washing purposes ; but these impedimenta are 

 not required with drv plates. Dry-plate processes 

 are of two principal lands : ( 1 ) Those in which the 

 collodion is applied to the glass plate, and after- 

 wards sensitised in the silver bath, as in the wet 

 way, but with a 'preservative' such as albumen 

 ' flowed ' over the surface, and the plate allowed to 

 dry. (2) Emulsion processes, in which the sensitive 

 silver salt in held in suspension in the collodion or 

 gelatine. A good emulsion can be prepared by 

 adding some soluble bromide, such as bromide of 

 cadmium, to the collodion, and afterwards an 

 alcoholic solution of nitrate of silver. The glass 

 plates are simply coated with this emulsion, washed 

 in water to remove the soluble salts, and set aside 

 to dry, when they are ready for use. The collodion 

 emulsion process is still employed to a limited 

 extent, chiefly for the production of transparencies ; 

 recent experiments indicate that it may yet compete 

 with gelatine emulsion in sensitiveness. 



The earliest form of the gelatino-bromide process, 

 at present so universally employed, appears to be 

 due to Dr Maddox, who published the details of 

 a workable emulsion of this nature in 1871. The 

 process was improved in 1873 by Mr Kennett, and 

 again in 1878 by Mr Charles Bennett. It was found 

 that if the emulsion was kept at a temperature of 

 90 F. for some days, or boiled for half an hour, the 

 sensitiveness of the plate coated by it was so greatly 

 increased that a view which could only be taken 

 formerly in 30 seconds could now be taken in one. 

 A very sensitive gelatine emulsion can also be 

 formed by using ammonia along with the bromide 

 of silver. Dry plates produced by some form of the 

 gelatino-bromide process are now manufactured on 

 a large scale. When properly made they will keep 

 good for years, and they can be developed months 

 after having been exposed. But it is generally 

 admitted that the best results are obtained when 

 the plates are not old, and when development 

 follows exposure without undue delay. 



PHOTOGRAPHIC PRINTING. Silver Prints. The 

 details of the well-known silver-printing process 

 are briefly as follow : 



( 1 ) Suitable paper is coated on the surface with a 

 smooth thin layer of albumen, to which chloride of 

 sodium or of ammonium has been added. (Originally 



