1 6-1 On the dlffereJit apfmrent Mag?iitudes of the same Ohjccts, 



seemed to me a great deal smaller than when T looked at it 

 directly. Since that time I have made repealled experiments 

 of this matter, and always found the same circumstances.*' 

 In another place he says : 



^* I shall recount still something more extraordinary on 

 this variation of the magnitude of the visual angle, or of 

 the image of objects. 



'S Last winter \ was in the country. In the night it froze 

 hard, and there fell a little snow. On going out of my 

 chamber in the morning, all objects appeared to me sensibly 

 smaller than they had done the evening before. 



^*. Since I made this discovery, and have been guard- 

 ed against the rule of comparison, T plainly perceive that 

 a very illuminated object seems smaller, arvd an object 

 feebly supplied with light appears 4arger, The reason of this 

 is evident. A strong liirht puts the whole globe of the eye 

 on contracting itself, and a feeble one leaves it relaxed and 

 dilated.'* 



This author is very correct in his observations ; but his 

 explanation is founded on a false theory. The true reason 

 is this : A strong light contracts the pupil oF the eye, in 

 which state it forms a small picture of an object upon the 

 retina; but in a weak one the pupil is ddated, and all ob- 

 jects then appear larger. This property of vision will, 

 I think, appear evident, from the following experiments. 



It is diflicult to enlarge the pupil of the eye to any parti- 

 cular dimension, but it may be contracted at pleasure, by 

 means oF perforations made either in a thin plate or a slip 

 of paper. 



Now, if an object be viewed through an -aperture, about 

 ...Vjj. of an inch in diameter, it will appear much smaller 

 than to the naked eye, in consequence of the aperture of 

 the crystalline lens being contracted ; but if the perforation 

 be removed from "l>efore the eye, the object will instantly 

 appear increased in magnitude: and as no change can take 

 place in any part of the eye instautajiemisl?/, it is therefore 

 evident, that, the apparent magnitudes of all objects arc in- 

 creased by an increase in the aperture of the crystalline lens, 

 and consequently by an enlarged pupil. 



4 The 



