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. 



