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Microscopical Essays 



pencils which proceed from the points o and b, will be received 

 on the lens F G, and by it carried parallel to the eye ; confe- 

 quently, the correfpondent points of the image Q P will be 

 vifible ; and thofe which are fituate farther out towards H and I, 

 will not be feen. This quantity of the image QP, or vifible area, 

 is called the field of view. 



Hence it appears, that if the image be large, a very fmall 

 part of it will be vifible ; becaufe the pencils of rays will for the 

 moft part fall without the eye glafs F G. And it is likewife plain, 

 that a remedy which would caufe the pencils, which proceed 

 from the extremes O and B of the object, to arrive at the eye, 

 will render a greater part of it vifible ; or, in other words, enlarge 

 the field of view. This is effected by the interpofition of a broad 

 lens D E (Fig. 5,) of a proper curvature, at a fmall diftance from 

 the focal image. For, by that means, the pencil DM, which 

 would otherwife have proceeded towards H, is refracted to the 

 eye, as delineated in the figure, and the mind conceives from 

 thence the exigence of a radiant point at O, from which the rays 

 laft proceeded. In like manner, and by a parity of reafon, the 

 other extreme of the image is feen at P, and the intermediate 

 points are alfo rendered vifible. On thefe considerations it is, 

 That compound microfcopes are ufually made to confift of 

 an objecT; lens L N, by which the image is formed, enlarged, 

 and inverted ; an amplifying lens D E, by which the field of view 

 is enlarged, and an eye glafs or lens, by means of which the eye 

 is allowed to approach very near, and confequently to view the 

 image under a very great angle of apparent magnitude. It is now 

 cuftomary to combine two or more lenfes together at the eye 

 glafs, in the manner of Euftachio Divini and M. Joblot; by 



which 



