222 Dr. North on the Angle of Aperture in Microscopes. 
* But it may perhaps be supposed from this reasoning, that if 
we throw a greater quantity of light upon an object, so that more 
may be collected by the object-glass, we shall be better able to — 
define a structure ; which would probably be the case if the ad- 
ditional light could be thrown only upon those minute parts of 
the object which we wish to examine, and not upon the whole 
object. But as we cannot do this, as the increase of illumination 
caunot be made to increase the relative proportions of light which 
proceed from those minute parts, the intended advantage will not 
be derived.” 
This paragraph involves implications directly opposite to some 
, of the most important facts in regard to using Microscopes suc- 
cessfully. The second sentence says in effect, that when, with 
an object-glass of small aperture, and in a faint light, we discover 
a certain degree of minuteness of structure, we shall, under 4 
strong illumination, discover no more, which is directly contrary 
act, unless the objective is miserably deficient in correc 
tion. Indeed, a most important quality of first rate objectives, 1s 
their ability to bear a strong illumination. 
It is also taken for granted that an increase of the relative propor 
tions of light upon minute’stria and other markings, would ena- 
bleus to see them better. On the contrary, the true requisite 1s 
that the objective shall be so perfectly corrected as to preserve Ihe 
relative light, shade, and variations of color on adjoimiug minute 
portions, thus exhibiting the object precisely as it would ap ked 
were its smallest parts /arge enough to be visible to the we 
a 
the 
hysiological than on optical conditions, being different with dif 
ferent individuals; one person’s eye being pained and dazzled, 
ed. 
notice that, although intrinsic brightness may be the same 
very intense, yet if all the light entering the pupil comes from 
a minute surface, the eye is less affected on the whole. Place 4 
ecard with a pin-hole at each end of a tube, and hold the ree 
near the eye, and we can gaze steadily at the small portion of t ‘ 
sun thus visible. But in general, the dazzling and blinding ene 
of intense light is owing to contrast, as when one comes IfOM” 
dark room into sunshine. The contrast may be of surrounding 
objects, as in viewing in a darkened room the light of a iene 4 
battery.* In reference to extreme intensity of illumination, : 
By means of this cont i . isplays may 8° 
times be made of lines aes hes pts Ne gah cna een Mr Wenham : 
i le conden: , the transverse lines - the ob 
(Greenport, in balsam,) are shown at regular intervals and sharply defined, °° 
jective being Smith & Beck’s +45, of largest angle. : - 
