ILLUMINATION OF OBJECTS. 



about a foot in diameter, having a hole in its centre, through 

 which the tube of the eye-piece is passed. This screen is then 

 at right angles to the axis of the body of the instrument, the- 

 eye-piece projecting about an inch from it. The observer looking 

 into the eye-glass with one eye, need not incur the exertion and 

 fatigue of closing the other, since the screen performs the office of 

 the eye -lid. 



The mirrors are sometimes made with a concave glass at one 

 side, and a plane glass at the other, the latter being used when 

 condensation is not required. A disc formed of plaster of Paris r 

 reduced to an extremely even and smooth surface, either plane 

 or concave, is sometimes used with advantage when a soft and 

 mild light is required. Nearly the same effect may be produced 

 by placing a disc of white card upon the face of the mirror. The- 

 illumination by a back light is attended with a peculiar advan- 

 tage, inasmuch as it displays the internal structure of objects, 

 and, in the case of organised bodies, supplies beautiful means of 

 exhibiting the circulation; as, for example, the circulation of the 

 blood in animals, and the sap in vegetables. In the case of certain 

 animalcules, it shows some living and moving within the bodies of 

 others. 



53. The following observations of Mr. Pritchard are worthy of 

 attention: " "We must consider that in all bodies viewed by 

 intercepted light, there is, properly speaking, neither light nor 

 shade, in the ordinary acceptation of these terms ; there are only 

 dark and light parts, which again assume new aspects as the light 

 is more or less direct or oblique. Thus depressions on transparent 

 objects are almost sure, under the action of oblique light, to 

 assume the effect of prominences ; but prominences seldom or 

 never have the semblance of depression. As almost all diaphanous 

 bodies can be examined as opaque objects, a scrutiny of them in 

 this way will generally be found greatly to assist our judgment 

 concerning their nature, whether they admit of being cut into 

 sections or not. It would be easy to write a volume on this 

 subject only, if we commenced an illustration of particulars which 

 could not be rendered clear and satisfactory without a vast number 

 of figures. Long practice must, after all, determine our opinions, 

 and scepticism should ever form a leading feature in them ; wo 

 should suspect rather than believe. 



" Opaque objects are not, upon the whole, so liable to produce 

 optical deceptions as transparent ones, because we are more in the 

 habit of viewing ordinary bodies by reflected or radiated light. 

 The most common illusion presented by them is that of showing a 

 basso-relievo as an alto-relievo ; the reverse deception sometimes 

 occurs also, but more rarely. This effect occurs in ordinary objects 



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