Intelligence and Miscellaneous Articles. 157 



quite a correct statement. He says that if the large mirrors are 

 kept parallel to the small ones, the " right " or natural relations 

 may be maintained between the dimensions of the object in the di- 

 rection of the depth of the field of view and those on the surface. 

 Now this is not ^possible with any arrangement of the mirrors ; for 

 the two retinal images being formed by visual rays which enter the 

 instrument at points considerably wider apart than the distance 

 between the eyes, must always have their difference so exaggerated 

 as to produce an exaggerated relief. It is true that when, in uni- 

 ting the two images, the optic axes are made to converge to a 

 distance considerably nearer than the object, the binocular image 

 appears reduced in size ; but this kind of reduction does not restore 

 the natural relation between the two dimensions, although an optical 

 reduction produced by concave glasses does. Perhaps it is the use 

 of concave glasses in his instrument that has led the Professor into 

 this mistake. But the fact is, that the reduction in size produced 

 by increased convergence of the eyes is not proportional in the nearer 

 and more distant j^arts of the same object; and hence, besides elon- 

 gation in depth, there is a distortion in figure ; thus a sphere always 

 appears through the instrument as an egg ivith its smallest end towards 

 the observer, whatever be the degree of optic convergence at which 

 its two images are united. 



The idea of combining my pseudoscope with the opera-glass so 

 as to correct the flatness of the binocular images given by the latter, 

 occurred to me very soon after my description of the pseudoscope 

 was published in 1853; and I now send you a rough sketch of a 

 stereoscopic opera-glass which I designed in November of the 

 same year. You will observe that it is just Professor Helmholtz's 

 Telestereoscope, with the object-glasses placed outside the large 

 mirrors ; the only other difference being, that I proposed to substi- 

 tute reflecting prisms with concave refracting surfaces for the small 

 mirrors and concave lenses. This sketch I sent with a note to 

 Mr. Adie, optician here, but he could not see a suSicient advan- 

 tage the instrument would have over the common opera-glass, to 

 warrant him in undertaking its manufacture as I suggested. He 

 returned the drawing, and it has lain by me ever since. 

 I am. Gentlemen, 



Your obedient Servant, 



Walter Haudie, 



[We have not considered it necessary to reproduce the sketch 

 forwarded by our correspondent, the difference being sufficiently 

 pointed out in the text. — En.] 



ON THE RELATIONS EXISTING BETWEEN CERTAIN GROUPS OF 



CRYSTALLINE FORMS BELONGING TO DIFFERENT SYSTEMS. 



BY C, MARIGNAC. 



It has long since been remarked that in each system the different 



forms arc not distributed by chance. They seem, on the contrary, 



to be combined in such a way as to form a certain number of groups. 



