[ have now mentioned the leading features 
of the human eye; you will all admit, I have = 2 @B 
no doubt, that it is extremely liable to imper- eS 
fection, and as such is the case, strict attention 
to details is demanded from the microscopist. 
Now although the human eye appears such Fig. 32. 
a wonderful instrument, there are many problems it is 
unable to solve without extraneoushelp. Many of 
you know perhaps the bunt of wheat, 7Zi//etia caries 
(fig. 32). With the unaided eye you will be able to 
discern nothing more than a black dust, the various 
| details having to be made out by other means. Then 
i again, with objects so minute as the diatom, Ampji- 
pleura pellucida (fig. 33), the object itself is almost 
. 
THE ELEMENTS OF MICROSCOPY.—THE HUMAN EYF. 303 
Binocular vision should be employed wher- 
ever practicable ; it will be found much less om 
trying to the eyes than monocular efforts. Re 3 
invisible to the unassisted eye, to say nothing of the 
beautiful carvings with which the valves are embel- 
lished, and which exact for their elucidation the most 
perfect lenses with which we are acquainted, and the 
most accurate manipulation of the illumination. You 
may indeed see the contour of many forms of dia- 
toms without extra optical assistance than that afforded 
us by nature, but not much more than this, as if the 
eye 1s approached too closely the picture falls behind 
the retina and is lost. 
I will now show you upon the screen a very much 
enlarged transparency of the diatom, P/eurosigma an- 
gulatum, which illustrates in a remarkable manner 
how errors of observation are likely to creep in. It is 
hard to believe at first that the white circles which 
you see are not hexagons, but are in fact true circles, 
which close investigation will prove. 
I have already mentioned the fact that starting with 
the distance of most distinct vision, continued ap- 
) proach to the eye finally renders the object invisible, 
| the rays being thrown de/ind the retina, the mechan- 
ism of accommodation being insufficient to produce 
a curve deep enough to bring the picture to a short 
conjugate focus. 
This can, however, be done by interposing a lens 
or lenses between the object and the cornea, so that 
a virtual image of the object is seen.’ How this 
affects the rays of light proceeding from an object 
I must leave to be explained in Part IL, which will 
deal with “Some of the properties of lenses.” 

Fig. 33. 
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a 

