OPTICS. 311 



lenses, one concave of white flint-glass between 

 two convex of crown-glass ; but still, where a 

 great magnifying power is wanted, recourse must 

 be had to reflecting telescopes. 



The greatest impediment to the construction of 

 large acromatic telescopes, is the want of a flint- 

 glass of an uniform density. Fortunately, for 

 Dollond, he met with a quantity of this kind of 

 glass when he began to make acromatic telescopes ; 

 but the attempts of many ingenious chemists have 

 since been exerted to make it, without much 

 success. 



The Multiplying Glass. 



The multiplying-glass is made by grinding down 

 the round side, h i k, of a convex glass A B, into 

 several flat surfaces, as h b, b d, die. An object C 

 (Plate 15. fig. 6.,) will not appear magnified, when 

 seen through this glass, by the eye at H ; but it 

 will appear multiplied into as many different objects 

 as the glass contains plane surfaces. For, since 

 rays will flow from the object C, to all parts of the 

 glass, and each plane surface will refract these rays 

 to the eye, the same object will appear to the eye 

 in the direction of the rays, which enter it through 

 each surface. Thus a ray C i H, falling perpen- 

 dicularly on the middle surface, will go through 

 the glass to the eye, without suffering any refrac- 

 tion ; and will, therefore, show the object in its 

 true place at C : whilst a ray c b, flowing from the 

 same object, and, falling ^obliquely on the plane 

 surface b h, will be refracted in the direction b e, 

 by passing through the glass ; and, upon leaving 

 it, will go on to the eye in the direction of e H ; 

 which will cause the same object C to appear also 



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