402 



NOTES AND QUERIES. 



[No. 291. 



Those writers, who have made the assertion on 

 Ware's authority, have utterly mistaken their 

 author ; for he mentions the rumour for the pur- 

 pose of refuting it. The whole was a trick of the 

 missionary priests, in order to produce divisions 

 in the English Church. On such slender grounds 

 does the assertion rest : and yet we find it re- 

 peated by one writer after another, until many 

 )>arsons actually receive the statement as an un- 

 doubted fact. T. L. 



THE PAKABOX OF VISION. 



Students in physical science need not be re- 

 minded that, in that branch especially which 

 relates to optics, certain paradoxical phenomena 

 have from the earliest times baffled the explanatory 

 attempts of writers upon these subjects. I allude 

 principally to the phenomena, or paradoxes as 

 they are commonly called, of single ^and inverted 

 vision, neither of which (to me at least) have been 

 satisfactorily explained in the various treatises, 

 popular or scientific, which have come beneath my 

 notice. With regard to the latter paradox — that 

 of seeing objects erect by inverted images on the 

 retina — first discovered by Kepler, and subse- 

 quently explained by Descartes, Smith, Berkeley, 

 Whewell, Brewster, Reid, &c., the attempted se- 

 lections have appeared to me (with all deference 

 to these great names) so vague, erroneous, and 

 confused, that I have been led to think that some 

 attempt at a more explicit and satisftxctory ex- 

 planation might not be unacceptable to the readers 

 of a miscellany, in the columns of which similar 

 questions have been discussed, and which professes 

 to be a " medium of scientific communication." 



The position of any external object is of two 

 kinds, absolute and relative. The absolute is its 

 actual position in space, considered without re- 

 ference to any other body. The relative is its 

 l)Osition considered with relation to some other 

 ■)odv. and is entirely independent of its absolute 

 position. 



Now nature has not endowed us with any fa- 

 culty whereby we are enabled to discover the 

 absolute position of a body in space ; nor can we 

 detect a change in such position, except by ob- 

 serving a corresponding change in relative position. 

 This we must lay down as our axiom, for it is 

 clearly the change in relative and not in absolute 

 position, which is made manifest to the senses ; 

 and if we are aware that a change has taken place 

 in the absolute position of any object, we must be 

 so simply by inference ; for our senses are utterly 

 inadequate to convey to the mind even the faintest 

 idea of such change. If a stone falls to the ground, 

 I perceive that it changes its relative position with 

 regard to the earth, and I infer that it has also 

 changed its absolute position in space. The ab- 



solute position of a man in space is continually 

 changing by the revolution of the earth on its 

 axis, yet he perceives no change for want of a fixed 

 standard whereby it can be made apparent. The 

 astronomer, indeed, has a standard in the sun ; and 

 were it not for this or some other, our change of 

 position from this cause would never have been 

 revealed to us. A man in a balloon, ship, or rail- 

 way carriage, cannot detect any change in his 

 absolute position, unless he fixes his eye upon 

 some stationary object, and he then perceives his 

 relative change, and infers that a corresponding 

 one is taking place in his absolute position in 

 space. The former alone is perceived ; we obtain 

 a knowledge of the latter by reasoning. 



Now the terms upright and inverted, as well as 

 all others which express the same idea, are purely 

 relative, and presuppose the existence of a cer- 

 tain standard of uprightness or inversion, without 

 which, indeed, they convey to the mind no idea. 

 We can attach no meaning to the expression " An 

 upright line," considered in itself, and remote 

 from all other lines and objects. An npriglit line 

 must be so with relation to something ; and what- 

 ever be its absolute position in space, it must 

 I'einain upright so long as its relation to that 

 something continues unchanged. In geometry, a 

 line which makes right angles with another right 

 line, is said to be perpendicular or upright, that is, 

 upright with respect to that other right line. It 

 would be equally so in every position so long as it 

 continues to make right angles with the line which 

 it touches. We might make these two lines re- 

 volve or invert them, as the images are said to be 

 inverted on the retina, without in any way de- 

 stroying the uprightness of the perpendicular line, 

 because we have previously established a test, or 

 standard of uprightness, which always attends it. 

 To destroy this quality of uprightness we must 

 alter the relation which the lines bear to each 

 other. We say that a man standing on his feet, 

 with his head pointing to the sky, is upright. 

 Here our standard is the earth. Now conceive 

 the man to be suspended in empty space, and the 

 force of gravity annihilated (for the direction of 

 gravity is a measuring standard), it is clear that 

 what position soever the man might there occupy, 

 he would always be in his natural and proper 

 position, or, in other words, every position would 

 be to him the same. In space there can be no 

 " uprightness," no " up," no " down ; " we may of 

 course fix upon a certain direction in space as our 

 standard of uprightness, and in that, case, if the 

 man were placed in a contrary direction, he might 

 with propriety be said to be inverted ; but he 

 would only be so in relation to that ideal standard. 



Now on the retina images are inverted only 

 with relation to their absolute and actual position 

 in space. They are not inverted with relation to 

 each other. The candle which points to the 



