field's and crouch's educational microscopes. 59 



mental requirement. In its general plan it conforms to the Ross 

 model ; the Body being carried upon an arm which contains the 

 lever for slow motion, acted on by a screw turned by a milled- 

 head ; and the Arm being carried by a bar which is raised by a 

 rack-and-pinion movement from the interior of the stem. This 

 stem, to which the Stage is attached above and the Mirror below, 

 is swung on transverse centres between two uprights that are cast 

 in one piece with the triangular foot. The Stage is furnished on 

 its upper surface with a movable brass ledge, against which the 

 object rests when the stage is inclined in any degree to the horizon ; 

 this ledge should slide smoothly and easily from the back to the 

 front of the stage, but should have at the same time sufficient hold 

 upon it to retain its position, and to support the object at what- 

 ever point it may be left. At a little distance beneath the stage 

 there is attached to it a Diaphragm -plate, perforated with holes 

 of various sizes for the regulation of the quantity of light admitted 

 to Transparent objects (§ 78), and also affording in one of its posi- 

 tions a dark back-ground, which is useful when Opaque objects 

 are being viewed. The stage is perforated at one of its front cor- 

 ners with a hole into which fits a pair of Stage-Forceps (§ 94). The 

 Mirror, which is concave on one side and plane on the other, is 

 attached, not directly to the stem, but to a tube which slides upon 

 it, so that its distance from the under-side of the stage may be 

 increased or diminished. The Condenser for opaque objects is 

 mounted on a separate stand (§ 90). — The siinpHcity of the con- 

 struction of this Microscope, and the facility with which all those 

 adjustments may be made that are required for the purposes it is 

 intended to fulfil, should constitute, with its low price, a great 

 recommendation to those who value a Microscope rather as a means 

 of interesting recreation for themselves, or of cultivating a taste 

 for the Study of Nature and a habit of correct observation in the 

 young, than as an instrument of Scientific research. It is not, of 

 course, to be expected that it should bear comparison, in regard 

 either to the mechanical finish of its workmanship, or to the per- 

 fection of its optical effects, with Microscopes of many times its 

 cost ; but it is infinitely superior to the best Microscope ever con- 

 structed on the old (non-achromatic) plan ; and it is greatly to be 

 preferred in its mechanical arrangements to any of the earlier 

 achromatic microscopes, which it at least equals in optical per- 

 formance.* 



42. Crouch? s Educational Microscope. — The instrument just de- 

 scribed cannot be made suitable to any higher purposes than those 

 for which it was originally devised. That now to be mentioned, on 



* Microscopes suited to Educational purposes, and corresponding more 

 or less closely in price and general arrangement with the above, are 

 made by Messrs. Baker, Collins, Highley, Steward, Wheeler, and other 

 Opticians in London, and by Messrs. Parkes, in Birmingham. In most 

 of these the workmanship is superior, but the variety of Powers and the 

 number of Accessories are less. 



