Prof. Helmholtz on the Telestereoscope . 19 



This heat applied to the metal would raise it 25133°. 



The weight of a molecule of mercury being 1 If times that of 



water, the cohesion integral of a luolecule is ^^ times that of 



a molecule of water, while its bulk is less in the proportion of 

 7-44 to 9. 



22 London Street, Edinburgh, 

 November 2, 1857. 



II. On the Telestereoscope. By Prof. H. Helmholtz*. 



THE image upon the retina of every human eye represents a 

 perspective projection of the objects situated in the field 

 of view. As the positions from which these projections are taken 

 are somewhat different for both eyes of the same individual, the 

 perspective images themselves are not identical ; and we make 

 use of their diflference, as stereoscopic experiments teach us, to 

 obtain an idea of the different distances of the objects represented 

 from the eye. Now the images of the same object on the two 

 retinae are the more different from each other the nearer the 

 object is to the eyes. In the case of objects, in comparison with 

 whose distance the space between the eyes is a vanishing quan- 

 tity, the difference between the two images also vanishes ; and 

 for such objects we lose the aid just spoken of, in estimating 

 their distance and bodily figure. 



The inspection of distant objects of irregular form, for example 

 of mountain ridges which close our field of view, will convince us 

 of this. The latter always appear to encircle us at the horizon, 

 like an erect wall : we perceive nothing of the protuberances, 

 indentations, nor of the different mountain chains which lie hid 

 one behind another, except through the help of shadows, aerial 

 perspective, or a more exact knowledge of their forms previously 

 obtained. With objects of irregular [regular ?] form, such as 

 l^uildings, &c., we are more aided by a simple perspective draw- 

 ing in forming a conception of those dinu-nsions which lie in 

 the direction of the depth of the picture. 



In the case of stereoscopic landscapes, which are now produced 

 to such an extent by photography, this defect is remedied by the 

 ])hotographer choosing for the taking of his pictures two suffi- 

 ciently distant positions, and thus ol)taining two sufficiently dif- 

 ferent projections of the landscape. The |)crson who views these 

 in the stereoscope, believes that he sees a reduced model of the 



♦ From I'ojrgcndorti 's Ainiahn, l!^.'i7, Nt>- ■'^• 



C3 



