ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 723 



tlie parts in relief appear hollow, and that which is hollow assumes 

 the appearance of perfect relief. It is indeed true that the illusion 

 is not always, nor with all persons, equally successful, and that some- 

 times the appearance is alternately that of hollow and of relief to the 

 same eye and with the same object ; but, in general, the inversion of 

 form does not lead to deception, not being able to overcome either 

 the knowledge of the object which the observer possesses nor that of 

 the reversal of the image brought about by the instrument. Physicists 

 admit that in this case the illusion proceeds from the observer's 

 knowing the direction from which the light comes, and seeing in the 

 image the lights and shades of the prominences or cavities on the 

 side opposite to that which, having regard to the direction of the light, 

 they ought to occupy ; so that, in the absence of any final test of the 

 comparison to aid the judgment, one argues from the position of 

 the lights and shades that what is really hollow is in relief, and vice 

 versa. In fact, if every part of the object is illuminated, or if (as 

 Brewster has suggested) a pin is placed upright by the side of it, and 

 one observes the direction of the shadow which it throws on the 

 object, the illusion suddenly vanishes and the object is seen as it 

 really is, and not as one's erroneous first impression had rej)resented 

 it. Almost all who have written upon the subject of vision, or the 

 illusions of the senses, refer to this curious phenomenon, and attribute 

 its discovery now to one, now to another person, according to the 

 patience, erudition, and perhaps the nationality of the writer; for, 

 with regard to the priority of discoveries, the factors on which the 

 final judgment depends are numerous. Joblot, in 1718, believed 

 himself to have observed it for the first time, not referring to any one 

 who had preceded him. Gmelin does the same in 1745, in a paper 

 on a kindred subject, printed in the ' Philosophical Transactions.' 



I do not know the purport of Rittenhouse's communication of 

 1786, because I have not hitherto succeeded in procuring the ' Trans- 

 actions of the American Philosophical Society,' which contains a 

 work by tliat author on some such subject, but it is probable that, like 

 Joblot and Gmelin, he too has believed himself to be the discoverer 

 of the phenomenon. Muncke, in 1828, in the article " Gesicht," in 

 Gehler's ' Dictionary of Natural Philosophy,' attributed the discovery 

 to Joblot (written Joblot by him), 



David Brewster, in publishing, in 1831, his 'Letters on Natural 

 Magic,' dedicated to Sir Walter Scott, alludes to an observation of this 

 nature made by the members of the Royal Society of London in one 

 of the first and earlier meetings of that society, and perhaps mentions 

 it as well in an article in the ' Edinburgh Scientific Journal,' which 

 I have been unable to consult. In 1838, Charles Wheatstone, in the 

 publication and description of his wonderful ' Stereoscope,' alludes to 

 the Royal Society of London as having first called attention to the 

 strange phenomenon, without, however, giving the year or stating 

 the miinnor in which it happened. Hclmholtz, in his ' Physiological 

 Optics,' reproduces Muncke's citations, and seems to adhere to Joblot 

 as the discoverer of the illusion. Schroder, writing on tlie subject in 

 1858, stops at Gmelin, and attributes the discovery to him. 



