194 



temperature 35. A thermometer was sent down with the weight 

 yesterday at 2000 fathoms, and returned with a minimum tempera- 

 ture of 38-5. Now 35 was the minimum temperature at 2700 

 fathoms in the Atlantic, further south than this cast, which was near 

 Rose Galley Rocks. I am therefore inclined to think that this is 

 about the minimum temperature of the great depths of the Ocean, 

 and that it commences soon after passing 2000 fathoms. 



III. " On The Stereomonoscope, a new Instrument by which 

 an apparently Single Picture produces the Stereoscopic 

 Illusion." By A. CLAUDET, Esq., F.R.S. Received 

 March 10, 1858. 



In a former paper " On the Phenomenon of Relief of the Image 

 formed on the ground glass of the Camera Obscura," which I com- 

 municated to the Royal Society on the 8th of May 1856, after having 

 investigated the cause of that extraordinary fact and tried to explain 

 it, I found that the images produced separately by the various 

 points of the whole aperture of an object-glass are visible only when 

 the refracted rays are falling on the ground glass in a line nearly 

 coinciding with the optic axes ; so that when both eyes are equally 

 distant from the centre of the ground glass, each eye perceives only 

 the image refracted in an oblique direction on that surface from the 

 opposite side of the object-glass. Consequently each side of an 

 object-glass, in proportion to its aperture, giving a different perspec- 

 tive of a solid placed before it, the result is an illusion of relief as 

 conspicuous as when looking naturally at the objects themselves. 



From the consideration of these singular facts, unnoticed before, 

 I was led to think that it would be possible to construct a new 

 Stereoscope, in which looking with both eyes at once on a ground 

 glass at the point of coalescence of the two images of a stereoscopic 

 slide, each refracted by a separate lens, we could see it on that 

 surface in the same relief which is produced by the common 

 stereoscope. 



This instrument, as may be perceived at once, is nothing more 

 than an ordinary camera obscura supplied with two lenses, each 

 mounted on a sliding frame in order to be able to give them, accord- 

 ing to the focal distance, the horizontal separation necessary for pro- 



