344 SCIENTIFIC AMUSEMENTS. 



back of the eyeball, where a diminished inverted image is formed. 

 This may be shown without much difficulty by obtaining from the 

 butcher a bullock's eye in a fresh condition, and very carefully 

 cutting and paring away the opaque harder portion of the external 

 coating. If this is done successfully, so that the remaining semi- 

 transparent part of the coating can act as a screen, an inverted 

 image of a candle flame, &c., can be readily seen to be formed on 

 holding the eye with the front part facing the candle. 



Short-sighted persons have naturally eyes of such a character 

 that the lens forms images of far off objects in front of and not 

 exactly upon the sensitive back part of the eye (or retina), by the 

 production on which of an image the sensation of vision is pro- 

 duced ; with nearer objects the image is formed further back, and 

 more nearly upon the retina, so that such persons can see objects 

 close to the eye much more distinctly than those at a greater 

 distance. A concave lens interposed between the eye and a far off 

 object causes the image to be formed further back than would 

 otherwise be the case ; - hence short-sighted persons require 

 spectacles where the glasses are thinnest in the middle, the degree 

 of concavity being proportionate to the defect in the eye. 



Long-sighted persons have the opposite defect; owing to 

 flattening of the crystalline lens through advancing age, or other 

 analogous causes, the image of a near object would be formed 

 further back than the retina, and consequently the object is only 

 seen indistinctly, whilst a far off object produces an image less far 

 back, and is therefore seen more clearly. In this case a convex 

 lens is requisite to cure the defect, the curvature, as before, being 

 suitably proportioned to the degree of flattening of the crystalline 

 lens. 



It frequently happens that both eyes are not alike, and that the 

 curvature requisite in the lens for the right eye is not the same as 

 that necessary for the left eye, in, order to produce equally distinct 

 vision with both. 



Combination of Lenses and Mirrors. Several kinds of optical 

 instruments are in common use, where the essential function of the 

 arrangement is to produce an image largely magnified, as compared 

 with the object viewed. According as the object is a long way off 

 or close at hand, the instrument is generally known as a telescope 

 (from the Greek for vision afar off) or a microscope (similarly from 

 the Greek for minute vision). In order to obtain the best defini- 

 tion, simple lenses, &c., are usually replaced by combinations of 

 lenses for the purpose of obtaining freedom from chromatic and 

 spherical aberration, &c.; but the general principles involved are 

 not affected by this circumstance. 



