NATURAL PHILOSOPHY. 199 







interpolated sand must have been taken. The station once fixed upon, it 

 was not difficult to hit upon the culprit in the small number of employees on 

 duty there. 



INVESTIGATIONS RELATIVE TO THE STEREOSCOPE. 



Motion in Stereoscopic Portraits. M. Lugeol, in making? the stereoscopic 

 portrait of one of his friends, had the idea of taking the two images or proofs 

 one after the other, and making his friend each time look upon a different 

 object. Thus, during the first sitting, he looked at the glass of the camera 

 obscura, and during the second to the right at an object fixed nearly at an 

 angle of 45 deg. These two images being placed in the stereoscope, let the 

 observer stand opposite a window, and, without ceasing to look at the por- 

 trait, turn himself to the left or right, he will see the eyes of the portrait follow 

 him as if they were animated. More than this has been effected by an adap- 

 tation of Sir David Brewster's natural magic toy, whose figures drawn on a 

 circle are moved quickly round, so that three or four of them catch our eye at 

 a particular angle, at almost the same instant, or rather at imperceptible 

 intervals, the effect of motion is given to the limbs of the stereoscopic portrait. 



On various Phenomena of Refraction through Semi-Lenses producing Ano- 

 malies in the Illusion of Stereoscopic Images. A paper was presented at the 

 last meeting of the British Association by Mr. Claudet, the well known pho- 

 tographist, having for its object to explain the cause of the illusion of cur- 

 vature given to pictures representing fiat surfaces, when they are ex- 

 amined in the refracting or semi-lenticular stereoscope. The author showed 

 that all vertical lines seen through prisms or semi-lenses are bent, present- 

 ing their concave side to the thin edge of the prism, and as the two photo- 

 graphic pictures are bent in the same manner, the inevitable result of their 

 coalescence in the stereoscope is a concave surface. The only means to 

 avoid this defect is to employ the centre of the lenses to examine the two 

 pictures ; but as the centre does not retract laterally the two images, their 

 coincidence cannot take place without placing the optical axis in such a posi- 

 tion that they are nearly parallel, as if we were looking at the moon, or a 

 very far object. This is an operation not very easy at the first attempt, but 

 which a little practice will teach us to perform. Persons capable of using 

 such a stereoscope will see the pictures more perfect, and ah 1 objects in their 

 natural shape. Mr. Claudet presented to the meeting a stereoscope made on 

 this principle, and many of the members present could see perfectly well with 

 it. The author explained the cause of a defect which is very often noticed in 

 examining stereoscopic pictures, viz. that the subject seems in some cases to 

 come out of the openings of the mountings, and in some others to recede from 

 behind this last effect being more favorable and more artistic. Mr. Claudet 

 recommended photographers when mounting their pictures to take care that 

 the openings should have their correspondent vertical sides less distant than 

 any two correspondent points of the first plane of the pictures, which could 

 be easily done by means of a pair of compasses, measuring those respective 

 distances. Mr. Claudet illustrated the phenomenon of vertical lines, by bent 

 prisms forming by coalescence concave surfaces. He stated that if holding in 



