OPTICAL PRINCIPLES OF THE MICROSCOPE. 21 



siderable 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 hign 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. 

 Many of the magnifiers sold as * Coddington Menses, however, are not 

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

 convex lenses, and are therefore destitute of the special advantages of the 

 real ' Coddington.' The * Stanhope' lens somewhat resembles the pre- 

 ceding in appearance, but differs 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 distances 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 but- 

 terflies' wings, and the other similar flat and minute objects, which will 

 readily adhere to the surface of 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, etc., etc. 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 enlarge- 

 ment of minute pictures photographed on its plane surface in the focus 

 of its convex surface. A good i Stanhoscope,' magnifying from 100 to 

 150 diameters, is a very convenient form of hand-magnifier for the recog- 

 nition of Diatoms, Infusoria, etc. ; 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. 1 



25. For the ordinary purposes of Microscopic dissection, single lenses 

 of from 3 to 1 inch focus answer very well. But when higher powers 

 are required, and when the use of even the lower powers is continued for 

 any length of time, great advantage is derived from the employment of 

 Achromatic combinations now made expressly for this purpose by several 

 Opticians. The writer has worked most satisfactorily for several years 

 with the 'platyscopic lens/ magnifying about 15 diameters, made by Mr. 

 Browning, who makes similar combinations of 20 and 30 diameters. 

 And he can speak equally favorably of the ' Steinheil doublets ' (con- 

 structed by the eminent Munich optician of that name, and introduced 

 into this country by Messrs. Murray and Heath), of which there are six, 

 ranging from 2 inches to inch focus. The Browning and the Stein- 

 heil combinations give much more light than single lenses, with much 

 better definition, a very flat field, longer working distance (which is very 

 important in minute dissection), and, as a consequence, greater ' focal 

 depth' or 'penetration' i. e. a clearer view of those parts of the object 

 which lie above or below the exact local plane. And only those who, 

 like the writer, have carried on a piece of minute and difficult dissection 

 through several consecutive hours, can appreciate the advantage in com- 

 fort and in diminished fatigue of eye, which is gained by the substitution 



1 See "Quart. Journ. of Microsc. Science," Vol. vi., N.S. (1866), 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. 



