1886.] 



On a New Form of Stereoscope. 



319 



picture projected from the left hand lantern, and the right eyes can 

 only see that from the right hand lantern. 



The rotation of the disks must be of such a rate, that the alternate 

 flashes of the right and left pictures on the corresponding eyes follow 

 in such rapid succession that the impression made by one flash does 

 not diminish sensibly before the next flash on the same eye is received. 

 The number of flashes for each eye which is required to produce an 

 apparently continuous view, without any flickering effect, is from 

 thirty to forty per second. As the disks are so cut as to produce two 

 flashes for the right eyes, and two for the left in one revolution, they 

 must consequently be kept rotating at a rate of from fifteen to twenty 

 revolutions per second. 



The rotation of the disks is effected by a driving-wheel and band, 

 * worked by a crank handle at the back of the apparatus. 



The perspective effect obtained by the above arrangement is very 

 perfect, the image of each object standing out in solid relief. 



Considering that by this arrangement the two eyes never see at the 

 same time, and that each eye views its picture after the other, it is 

 interesting to find that the persistence of vision so completely bridges 

 over the alternate interruptions to which it is subjected as to produce 

 the effect of a continuous view. 



An unavoidable effect resulting from this arrangement is, that by 

 the rotation of the disks one half of the light produced by each 

 lantern is always cut off; the higher, therefore, the illuminating 

 power used the better is the result. 



This defect is, however, I consider, counterbalanced by several 

 advantages which this form of stereoscope possesses. Firstly, the 

 pictures can be enlarged to such an extent as to appear equal, or even 

 larger than the original objects from which they were taken ; and 

 secondly, the eyes, in looking at the pictures, are not in any way 

 subjected to strain by lenses, prisms, or reflectors, or by the difficulty 

 which some persons experience in getting the two pictures to super- 

 pose. For each eye views its corresponding picture in exactly the 

 same position it would see it in if it were looking at the original, 

 since the two pictures are practically in the same place, which is not 

 the case in any other form of stereoscope. 



Although with the apparatus as here described only two persons 

 can see the pictures at the same time, it would not be very difficult to 

 construct it so as to be available for a greater number. The side disks 

 above described only serve to control one pair of eye-holes each, but 

 by making them a little larger they would serve for two pairs each, 

 thus accommodating four observers. By increasing the number of 

 disks, the number of observers might be increased proportionately. 



VOL. XL. 



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