Prof. Emerson on the Perception of Relief. 125 



annual inequality independent of latitude would exist if the sun's 

 equator coincided with the ecliptic, the inequality in question 

 must be attributed to the inclination of the solar equator, in 

 consequence of which the effect of the gyratory motions about 

 the sun will be at a maximum each time the earth passes through 

 the nodes, that is, as we have seen, about the times of the summer 

 and winter solstices. 



There is another phenomenon similar, and perhaps related to 

 the zodiacal light, which is described by Mr. Jones in a second 

 letter, dated from Quito, Nov. 18, 1856, and published in vol. v. 

 of the ' Astronomical Journal ; (p. 28). He says, "I see here 

 every night, and all through the night, a luminous arch, from 

 east to west, quite across the sky.-" The arch is also stated to 

 be 20 degrees broad, and of uniform intensity throughout ; and 

 apparently it stretches along the ecliptic, although this is not 

 expressly mentioned. The phenomenon described by Prof. 

 Brorsen of Senftenberg, in Nos. 998 and 1166 of the Astrono- 

 mische Nachrichten, appears to be identical with this. I have 

 purposely abstained from any attempt to account for this lumi- 

 nosity, because it seems to require for its explanation considera- 

 tions distinct from those which apply to the zodiacal light, 

 although, like the latter, it is in some way related to the sun. 



Cambridge, January 16, 1863. 



XVI. On the Perception of Relief . 

 By Professor Edwin Emerson, of Troy University , £/".£.* 



PROFESSOR CIMA of Turin has sent us the description (says 

 the editor of the Cosmos) of a stereoscopic experiment which 

 is not without interest. He takes the picture of a front view of a 

 human head, executed either in crayon, or lithograph, or copper- 

 plate, and which is 3 or 4 centimetres in height ; this he cuts 

 into two parts along a line which coincides with the vertical axis 

 of the nose ; he takes one of these halves in each hand, and hold- 

 ing them in the same perpendicular plane, he brings them before 

 the eyes at a distance which is less than that of distinct vision ; 

 he then allows the optic axes to converge, and thus causes the 

 drawings to approach or recede until he is able to see two pictures 

 of each half, and until the two middle ones overlap so that they 

 make the impression of an entire countenance. " When one makes 

 this experiment for the first time," says Professor Cima, " he will 

 see with astonishment that the full face which is produced by 

 the overlapping of the two halves makes, in a high degree, the 

 impression of a solid body ; the half tones melt and mix together 



* From Silliman's American Journal for November 1862. 



