STANHOPE LENS — COMPOUND MICROSCOPE. 81 



upon by Mr. Coddington, who pointed out that the same end 

 would be much better answered by taking a sphere of glass, and 

 grinding a deep groove in its equatorial part, which should- be 

 then filled with opaque matter, so as to limit the central aper- 

 ture. Such a lens gives a large field of view, admits a considera- 

 ble amount of light, and is equally good in all directions ; but 

 its powers of definition are by no means equal to those of an 

 achromatic lens, or even of a doublet. This form is chiefly use- 

 ful, therefore, as a hand magnifier, in which neither high power 

 nor perfect definition is required ; its peculiar qualities rendering 

 it superior to an ordinary lens of the same power, for the class 

 of objects for which such lenses are applied in this mode. We 

 think it right to state that many of the magnifiers sold as " Cod- 

 dington" lenses are not really (as we have satisfied ourselves) 

 portions of spheres, but are manufactured out of ordinary double- 

 convex lenses, and will be destitute, therefore, of many of the 

 above advantages. It may be desirable to allude to the magni- 

 fier known under the name of the " Stanhope" lens, which some- 

 what resembles the " Coddington" in appearance, but difiers 

 from it essentially in properties. It is nothing more than a 

 double-convex lens, having two surfaces'of unequal curvatures, 

 separated from each other by a considerable thickness of glass ; 

 the distance of the two surfaces from each other being so ad- 

 justed, that when the most convex is turned towards the eye, 

 minute objects placed on the other surface shall be in the focus 

 of the lens. This is an easy mode of applying a rather high 

 magnifying power to scales of butterflies' wings and other similar 

 flat and minute objects, which will readily adhere to the surface 

 of the glass ; and it also serves to detect the presence of the 

 larger animalcules, or of crystals in minute drops of fluid, to ex- 

 hibit the " eels" in paste or vinegar, &c. &c. ; but it is almost en- 

 tirely destitute of value as an instrument of scientiflc research, 

 and can scarcely be regarded in any higher light than as an in- 

 genious philosophical toy." 



20. Compound Microscope. — ^In its most simple form, this instru- 

 ment consists of only two lenses, the " object-glass" and the " eye- 

 glass :" the former, c d (Fig. 11), receiving the rays of light direct 

 from the object, A B, which is brought into near proximity to it, 

 forms an enlarged and inverted image a' b' at a greater distance 

 on the other side ; whilst the latter, L m, receives the rays which 

 are diverging from this image, as if they proceeded from an 

 object actually occupying its position and enlarged to its dimen- 

 sions, and these it brings to the eye at e, so altering their course 

 as to make that image appear far larger to the eye, precisely as 

 in the case of the simple microscope (§ 16). It is obvious that 

 by the use of the very same lenses, a considerable variety of 

 magnifying power may be obtained, simply by altering their 



' The principal forms of construction of Simple Microscopes, will be described in the 

 next chapter. 



6 



