336 Prof. Neckei' 07i a7i apparent Change of Position 



however, the sun has also been setting for these higher re- 

 gions of the atmosphere, the contrast between their illumina- 

 tion and the shade existing all over Mont Blanc, to which 

 was owing that blueish and deadly colour assumed by the 

 eternal snows, having ceased to take place, the Mont Blanc 

 assumes once more, but in a much fainter and darker manner, 

 its orange yellow colour ; and the lower and nearer moun- 

 tains recover their purplish hue. All the objects then being 

 uniformly and altogether illuminated by the much paler and 

 less powerful light of the twilight, as they were before all 

 lighted at once by the brighter, but equally uniformly spread, 

 light of the sun; so that every thing being placed in the same 

 relative quantity and quality of illumination as before, an 

 analogous aspect is seen in both cases, though much darker 

 under the latter than under the former circumstances. Hence 

 it appears to me that the whole of the phaenomenon is most 

 naturally and easily explained by contrast. 



The object I have now to call your attention to, is an ob- 

 servation which is also of an optical nature, and which has 

 often occurred to me while examining figures and engraved 

 plates of crystalline forms: I mean a sudden and involuntary 

 change in the apparent position of a crystal or solid repre- 

 sented in an engraved figure. What I mean will be more 

 easily understood from the figure annexed. The rhomboid 

 AX is drawn so that the solid angle A should be seen the 

 nearest to the spectator, and the solid angle X the farthest frona 

 him, and that the face ACBD should be the foremost, while 

 the face XDC is behind. But in looking repeatedly at the same 

 figure, you will perceive that at times the apparent position of 

 the rhomboid is so changed that the solid angle X will appear 

 the nearest, and the solid angle A the furthest; and that the 

 face ACDB will recede behind the face 

 XDC, which will come forward ; which 

 effect gives to the whole solid a quite con- 

 trary apparent inclination. I have been 

 a long time at a loss to understand the ^ ^ 



reason of the apparently accidental and 

 involuntary change which I always witnessed in all sorts of 

 forms in books of crystallography. The only thing I could 

 observe was, that at the time the change took place, a parti- 

 cular sensation was felt in the eye (for it takes place as well 

 when seen with only one eye, as with both eyes), which proved 

 to me that it was an optical, and not merely as I had at first 

 thought a mental, operation which was performed. Atter, how- 

 ever, a more attentive analysis of the fact, it occurred to me, 

 that it was owing to an involuntary change in the adjustment 



