OPTICAL IMAGES. 
the natural appearances and effects which are familiar to every 
eye, and innumerable contrivances, from which we derive 
essential benefit, either in repairing defects of vision, or ex¬ 
tending the range of that sense to objects removed beyond its 
natural limits, either because of their minuteness or remoteness, 
or in fine in producing phenomena affording at once amusement 
and instruction. 
The landscape seen inverted in the tranquil surface of the river 
or lake ; the ship seen reproduced in like manner in the face of a 
calm sea ; our persons, and the objects which surround us, seen in 
a looking-glass ; the clear vision conferred on weak eyes by one 
sort of spectacle-glass, and the distinct vision conferred on strong 
but short-sighted eyes, by another; the apparent enlargement 
produced by magnifying glasses; the clear view of the scene and 
its personages afforded by the opera-glass; in fine, the mar¬ 
vellous world of minuteness opened to our view by the microscope, 
and the sublime spectacle of the remote regions of space, teeming 
with countless systems of suns and circumvolving worlds, dis¬ 
played before us by the telescope, are a few, and only a few, of 
the innumerable things of wonder and interest, to comprehend 
which is impossible without some knowledge of the manner in 
which optical images are produced. 
As we shall, from time to time, present all these interest¬ 
ing subjects in the pages of the “ Museum,” we propose now, 
as an indispensable preliminary, to explain with as much 
brevity as may be compatible with clearness, the principles 
Upon which the natural and artificial production of optical images 
depends. 
2. It is, in the first place, and above all things, necessary to 
understand the manner in which the eye obtains the perception of 
any visible object, because if we can show that precisely the same 
means are called into operation in the case of an optical image, we 
shall understand how the latter produces the same sensible impres¬ 
sion as the object itself. 
To comprehend this, then, it is necessary to consider that each 
point of a visible object is a focus from which rays of light diverge 
exactly as if the point were luminous. Some of these divergent 
rays are received by the eye, and enter it through the circular 
hole called the pupil, * and there produce a perception of the point 
of the object from which they have radiated. Since each point of 
the object is thus a distinct focus, or centre of radiation, a percep¬ 
tion of each point, and therefore of the whole object, is thus 
produced. 
* See Tract on The Eye, vol. v., pp. 54, 55, 
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