62 KEPORT — 1860. 



nothing more than to give or restore to these images the natural angle at which the 

 objects are seen when we approach them or recede from them. For magnifying or 

 diminishing the size of objects is the same thing as approaching them or receding 

 from them, and in these cases the angles of perspectives cannot be the same. Mr. 

 Claudet showed that, looking at the various rows of persons composing the audience, 

 with the large ends of the opera-glass, all the various rows appeared too close to 

 one another, that there was not between them the distance or space which separates 

 them when we look with the eyes alone ; and he showed also that, with the small 

 end, the distance appeared considerably exaggerated. But applying the sets of 

 prisms to the opera-glass in order to increase the angle of the two perspectives, then 

 looking at the audience as before, it appeared that the various rows of persons had 

 between them the natural separation expected for the size of the image or for the re- 

 duction of the distance of the objects. By applying the two sets of prisms before 

 the eyes without the opera-glass, it was observed, as was to be expected, that the 

 stereoscopic effect was considerably exaggerated, because the binocular angle was in- 

 creased without magnifying the objects. But looking with the two sets of prisms 

 alone at distant objects, the exaggeration of perspective did not produce an unplea- 

 sant effect. It appeared as if we were looking at a small model of the objects 

 brought near the observer. By the same reason, stereoscopic pictures of distant 

 objects (avoiding to include in them near objects) can advantageously be taken at a 

 larger angle than the natural angle, in order to give them the relief of which they 

 are deprived as much when we look at them with the two eyes, as when we look 

 only with one eye ; instead of being a defect, it seems that it is an improvement. 

 In fact, the stereoscope gives us really two eyes to bring out in relief the pictures of 

 distant objects. 



On the Principles of the Solar Camera. By A. Claudet, F.R.S. 

 The solar camera, invented by Woodward, is one of the most important improve- 

 ments introduced in the art of photography since its discovery. By its means 

 small negatives may produce pictures magnified to any extent; a portrait taken on a 

 collodion plate not larger than a visiting card can be increased, in the greatest per- 

 fection, to the size of nature ; views as small as those for the stereoscope can be also 

 considerably enlarged. This is an immense advantage, which is easilv understood 

 when we consider how much quicker and in better proportion of perspective small 

 pictures are taken by the camera obscura, while the manipulation is so greatly sim- 

 plified. There is nothing new in the enlargement of photographic pictures. This 

 has been done long ago simply by attending to the law of conjugate foci ; and every 

 photographer has always been enabled, with his common camera, to increase or 

 reduce the size of any image. For the enlargement, it was only necessary to place 

 the original very near the camera, and to increase in proportion the focal distance. 

 But the more the focal distance was increased, the more the intensity of light was 

 reduced ; and a still greater loss of light arose from the necessity of diminishing the 

 aperture of the lens, in order to avoid the spherical aberration. Such conditions 

 rendered the operation so long that it became almost an impossibility to produce any 

 satisfactory results when the picture was to be considerably enlarged. For these 

 reasons, it naturally occurred, that if the negative, having its shadows perfectly 

 transparent and its lights quite black, was turned against the strong light of the sun, 

 its image at the focus of the camera would be so intense that the time of exposure 

 would be considerably reduced ; so that, in order to employ the light of the sun, 

 and follow easily its position without having to move constantly the whole camera, 

 it was thought advisable to employ a moveable reflecting mirror sending the parallel 

 rays of the sun on a vertical plano-convex lens, condensing those rays on the nega- 

 tive (placed before the object-glass and behind the condenser) somewhere in its 

 luminous cone. Many contrivances for this object were resorted to, but without 

 considering anything else than throwing the strongest light possible on the negative 

 to be copied. The constructors of these solar cameras never thought it very im- 

 portant to consider whether the focus of the condensing lens was better to fall before 

 or behind the front of the object-glass, provided the negative was placed in the lumi- 

 nous cone of the condenser. This want of attention has been the cause which has 

 made the solar camera a very imperfect instrument for copying negatives. The 



