OPTICAL DECEPTION. an 
looking at, the deception vanifhes as well as in the former 
cafe. The application of a writing pen or pencil will pro« 
duce the fame effect. And, which is very remarkable, 
after the mind has been undeceived by thefe means once 
or twice, it does not readily admit of the impofition again: 
Though, as I obferved before, if it be done by removing 
the glaffes, the deception will return again as often as you 
pleafe. The truth feems to be, that the mind chufes the 
leaft difficulty; and though in confequence of the judg- 
ment it has formed concerning the direction of the light, 
it will fubmit to fuch a fmall impofition as to fuppofe one 
piece of chocolate may have ridges where others ufually 
have furrows, when indeed it has not, yet it will not rea- 
dily endure fuch a grofs one, astofuppofe it to have cavities 
of the figure and colour of a finger or a writing pen. Or 
perhaps the vifible motion attending fuch application pro- 
duces the principal effect in convincing the mind that thofe 
bodies are really elevated*, and then their fhades and mo- 
difications of the light, fhew in what diredtion it falls on 
them; and the miftake of the mind in that particular be- 
ing rectified, the whole object muft affume its natural ap- 
pearance. 
The explanation I have given of this phenomenon will. 
account for an odd circumftance mentioned (I think) by 
Mr. Short; which once appeared fo whimfical to me as nei- 
ther to merit credit or attention. Mr. Short carefully ex- 
amined the Caflegrain telefcope, and in all probability fet 
it by the fide of one of the Gregorian form, in order to de- 
termine its comparative merits: He gives the preference 
to the Gregorian, and mentions asa principal defect of the 
Caffegrain telefcope, that it reprefents the mountains in the 
moon as vallies, and the contrary. I doubt not but this, 
¥ otherwife 
* Whilft I was making thefe experiments, I thought of a carved filver fhoe buckle, as a very 
proper object to prevent a deception of this fort from taking place. But placing it on a brick 
pavement, and looking at it through the glaffes, it neverthelefs appeared perfectly depreffed. 
Precifely as if you had taken a buckle and ftrewing on it a white fhining powder, had prefled 
it into the brick whilft foft, and then removing the buckle, the glittering powder had remained 
in the impreffion. 
