22 OPTICAL PRINCIPLES OF THE MICROSCOPE. 



it was given by Dr. Wollaston, who proposed to apply two plano- 

 convex or hemispherical lenses by their plane sides, with a - stop ' 

 interposed, the central aperture of which should be equal to l-5th 

 of the focal length. The great advantage of such a lens is, that 

 the oblique pencils pass, like the central ones, at right angles to 

 the surface ; and that they are consequently but little subject 

 to aberration. The idea was further improved upon by Sir 

 D. Brewster, 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 aperture. Such a lens gives 

 a large field of view, admits a considerable amount of light, and is 

 equally good in all directions ; but its power of definition is by no 

 means equal to that of an achromatic lens, or even of a doublet. 

 This form is chiefly useful, 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, for the 

 class of objects for which a hand -magnifier of medium power is 

 required. It should be stated, however, that many of the mag- 

 nifiers sold as ' Coddington ' lenses are not really portions of 

 spheres, but are manufactured out of ordinary double-convex 

 lenses, and are destitute, therefore, of many of the above advan- 

 tages. — The 'Stanhope' lens somewhat resembles the 'Cod- 

 dington ' in appearance, but differs from it essentially in proper- 

 ties. — 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 adjusted, 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 exhibit the 'eels' in paste or vinegar, &c. &c. — A modified 

 form of the ' Stanhope ' lens, in which the surface remote from the 

 eye is plane instead of convex, has been brought out in France 

 under the name of ' Stanhoscope, ' and has been especially applied 

 to the enlargement of minute pictures photographed on its plane 

 surface in the focus of its convex surface. A good ' Stanhoscope,' 

 magnifying from 100 to 150 diameters, is the most convenient form 

 of Hand-magnifier for the recognition of Diatoms, Infusoria, &c. ; 

 all that is required being to place a minute drop of the liquid to 

 be examined on the plane surface of the lens, and then to hold it 

 up to the light.* 



* See " Quart. Journ. of Microsc. Science," Vol. vii., N.S., p. 263. — Of 

 the Stanhoscopes sold by Toy-dealers at a very low price, only a part 

 are really serviceable ; care is requisite, therefore, in the selection. 



