MICROSCOPIC PHOTOGRAPHY.] UNDULATORY FORCES. LIGHT PHOTOGRAPHY. 



1G3 



cause of single vision by two eyes, may be taken as 

 descriptive of the effect of the stereoscope. The parti- 

 cular form of the instrument varies according to the 

 peculiar method of construction of each maker ; and 

 into such we shall not enter, but rather confine our- 

 selves to the method of taking stereoscopic pictures, and 

 the instrument required for that purpose. 



In a single camera, the rays of light passing from an 

 object to the ground glass or sensitive plate, do so 

 exactly as occurs when any object is seen by one eye. 

 If another camera be placed next to the first, the rays 

 will not fall exactly to the centre of its plate, because 

 they arrive at two different angles from the source. 

 Again, if an object, at a short distance from the eye 

 of the observer, is viewed alternately by each eye, it 

 will not appear in the same position as it would if 

 viewed by both eyes simultaneously. The distance be- 

 tween the two eyes, of course produces a separate 

 angle of vision for each ; and hence the right eye can 

 see further to the left than can the left eye ; and, vice 

 verfa, the left eye can see more to the right of any 

 object. 



Now stereoscopic pictures are always taken in dupli- 

 cate, each at an angle varying with the other, as the 

 angle of vision of the two eyes differs. It follows, 

 therefore, that either two cameras must be employed, or 

 that, if the picture is taken by one camera, then the 

 instrument must be successively placed in two different 

 angles to take the picture in a proper manner. An 

 inspection of any stereoscopic print will illustrate the 

 remarks we have here made ; for it will be found, that 

 one either exceeds or falls short of the other in the 

 representation of the width of its field of view. 



In practice, a double camera is always employed ; for 

 the operator, having the two instruments arranged per- 

 manently at the proper angle, has no trouble in the 

 mechanical part of the process. Besides this advan- 

 tage, the person desiring a likeness has not to undergo 

 the fatigue of a double sitting, and the chance of 

 placing the head and body in such a position as would 

 be required to produce a second and corresponding one 

 to that first procured by the single arrangement. 



The entire manipulation, beyond the management of 

 the camera, is exactly the same, in all respects, as that 

 we have described under the collodion process ; two 

 plates being employed, or rather one plate, on which 

 two pictures are produced. 



The stereoscope has done much towards popularising 

 the art of photography ; and the favour with which the 

 simple instrument has been received by the public, has 

 induced the invention of a great variety of arrange- 

 ments, by means of which, a number of pictures may 

 be successively viewed by one or more persons. The 

 pictures are generally mounted on an endless band, be- 

 tween two stereoscopes placed opposite to each other; 

 and thus the number of pictures is limited only by the 

 length of the band, or the size of the whole arrange- 

 ment. A handle, affixed to the roller on which the 

 band revolves, brings the objects successively in view 

 to either stereoscope. 



MICROSCOPIC PHOTOGRAPHY. 



SOME very interesting results have been obtained by 

 applying the photographic process to obtaining pictures 

 of microscopic objects. This is easily effected by re- 

 ceiving, either through or without the eye-piece of an 

 achromatic compound microscope, the image of the 

 object on a plate by means of the usual collodion pro- 

 cess. In fact, the lenses of the microscope answer in 

 place of the lens of the camera : the plate, however, 

 must be so arranged as to be entirely protected from 

 external light. This is easily done by placing it in a 

 small dark box at the end of the eye-piece, from which 

 the image proceeds to the prepared plate. 



Such pictures can afterwards be submitted to the 



inicroscojii;, and may of course be magnified to any 



d extent. One grca^ advantage arising from this 



use of photography is, that perishable objects can be 



copied exactly as they appear at any moment, and so 

 all their peculiarities of construction or constitution 

 may be permanently retained ; and if negatives are 

 taken, any number of copies can be subsequently pro- 

 duced either on glass or paper. 



Perhaps the most interesting kind of photographs of 

 this kind, are those which have been taken of the blood 

 globules of animals, of which we have already spokeu 

 in connexion with the oxy-hydrogen microscope.* \Ve 

 believe that these were first exhibited by M. Duboscq, 

 of Paris, at a meeting of the British Association, some 

 six years ago, at Glasgow. Since then, an immense 

 variety of microscopic photographs have been produced ; 

 and they are now a common article of sale at the 

 opticians, etc. 



PHOTOGRAPHIC REGISTRATION. 



AMONGST the various uses to which photography has 

 been applied, is that of registering magnetic, astrono- 

 mical, barometrical, and other observations. As each 

 of its applications is made in a form suitable to the 

 instrument, the indications of which have to be noted, we 

 shall confine our remarks to the explanation of the 

 general plan, rather than to separate details. 



A piece of paper rendered sensitive to the action of 

 light by any of the processes mentioned in the previous 

 pages, is fixed round a cylinder. This is moved by clock- 

 work, so as to rotate once in twelve or twenty-four 

 hours ; and the paper is divided into parts, so that 

 each hour or part of an hour is marked on its surface. 

 The whole is excluded from light, except at a small 

 aperture, through which the rays of a lamp, concentrated 

 by means of a reflector, alone can pass. Now the light 

 so admitted will at once act on the paper opposed 

 thereto ; and as the paper is continually revolving as 

 the day passes on, any ray falling on the surface will 

 produce a black mark at a place on the paper, indi- 

 cating the time of day. 



When the paper is removed, a long and irregular line 

 will be observed on the surface ; and the position of 

 this line will thus indicate say the vibration of a 

 magnetic needle, the rise or fall of the mercury in the 

 barometer tube, <tc. , according to the mechanical and 

 other arrangements to which it has been adapted. 



It will be at once seen that this plan is of the greatest 

 possible value to the philosopher. Day or night, the re- 

 gistration can be carried on with the most extreme 

 accuracy : no chance of error need creep in if proper 



Cautions are taken ; and the results, printed liy the 

 d of nature, are permanent, and not like the elforts 

 of the human mind, which are intermittent from the 

 necessity of rest, or uncertain from their infirmity. 

 The plan is extensively employed in most of the public 

 and private observatories of our own and other countries 



COLOURING PHOTOGRAPHS. 



WE regret to say, that all attempts to obtain pictures 

 with natural colours, by means of photography, have, as 

 yet, been unsuccessful. The photographic artist has, 

 therefore, endeavoured to supply this want by applying 

 colours to photographs, and so producing a picture re- 

 sembling that produced by the painter. 



The practice was first commenced with Daguerreo- 

 types, which, from their heavy metallic hue, were any- 

 thing but flattering likenesses in many cases. After the 

 collodion process was introduced, the art of colouring 

 was still more largely practised, and has now arrived, 

 in many instances, to a high artistic rank ; although, 

 when in the hands of the majority of those who employ 

 it, the results are scarcely better than hideous daubs, in 

 which the value (if any) of the photograph is entirely 

 concealed, and its defects strongly magnified. 



The colours, in a state of impalpable powder, are 



applied by means of a camel-hair pencil, moistened with 



a little water. Some highly-finished oil portraits have 



been produced by leading photographers ; which have the 



See ante, p. 6s. 



