of Natural Philosophy:. 145 
The statements concerning the brightness of the image, 
dado in different propositions of this chapter, are not legit- 
imately proved; for the number of rays received by the 
pupil from any one point of the object may be increased, 
and the brightness nevertheless diminished,——on account of 
the increase of apparent magnitude. 
Props. 108, 110, and 111, assert unconditionally con- 
cerning the magnifying power of mirrors, what is true only 
in certain positions of the eye. If, for example, the object 
be nearer a concave mirror than its principal focus, and the 
aye be in the centre Seg concavity, the image, instead of 
‘appearing larger’’ than the object, as is asserted in prop. 
108, will appear of the same magnitude; and if the eye be 
pone arate nearer ithe. smienenty ees mines ma appear, tbe 
aoe 144. Schol. re « Of two. refracting telescopes which 
magnity equally, the shorter will give a more imperfect im- 
age than the longer. For the image appearing equal i in both, 
but being farther from the object-glass in 8 _— than 
the shorter, must be in reality larger or more magnified ; 
whence the defect arising from the different rofraniibility of 
the rays, Wi be more visible in the longer than in the short- 
er telescope.” —The statement with which this paragraph 
egins is correct. The reasoning subjoined is. evidently 
erroneous, and leads to a conclusion the reverse of what 
was first asserted. If two ee were boa maoce: 
che a account ‘of lead et, lenses:in sr aa 
eat omits the essential circumstance on which the whole 
explanation.tarns. We are told that a convex lens of crown 
glass is to be united with a concave one of flint glass in such 
a manner “ the excess of refraction in the crown glass 
may colour caused by the flint glass.” Here 
the student will naturally i eine, aa the crown glass ean 
o. ¥. 
