38 VISION WITH THE COMPOUND MICROSCOPE 



considerable thickness of glass, the distance of the two surfaces from 

 each other being so adjusted that when the more 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. A modified form of the ' Stan- 

 hope ' lens, in which the surface remote from the eye is plane instead 

 of convex, lias been brought out in France under the name of 

 ' Stanhoscope,' and has been especially applied to the enlargement of 

 minute pictures photographed 011 its plane surface in the focus of its 

 convex surface. A good ' Stanhoscope,' magnifying from 100 to 150 

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

 recognition of diatoms, infusoria, tfcc., 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. But no hand 

 lenses we have yet seen will compare with the Steinheil ' loups ' of 

 six and ten diameters made by Zeiss, and Keichart's pocket loups. 

 For the ordinary purposes of microscopic dissection single lenses 

 of from 3 inches 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 Steinheil 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 have carried on a piece of minute and difficult dissection 

 through several consecutive hours can appreciate the advantage in 

 comfort and in diminished fatigue of eye which is gained by the 

 substitution of one of these achromatic combinations for a single 

 lens of equivalent focus, even where the use of the 

 former reveals no detail that is not discernible by the 

 latter. 



Although not strictly its position, it is convenient 

 here to refer to what is known as the ' Briicke lens ; ' 

 it is much used on the Continent, but does not ap- 

 pear in any English treatise we have seen. It has 

 two achromatic lenses for the objective, and a concave 

 eye lens. It is illustrated in fig. 29. 



To remedy the inconvenience of the lens being too 



close to the object in all but low powers, Charles 



FIG 29 The Chevalier, in his 'Manuel du Micrographe' (1839), 

 Briicke lens, proposed to place above a doublet a concave achro- 

 matic lens, the distance of which could be varied at 

 pleasure. The effect of this combination is to increase the magnifying 

 power and lengthen the focus. Thus arranged, this instrument will 



