UNDULATORY FORCES. LIG IIT. 



[COMPOUND MICROSCOPE. 



a shock from it in the usual manner, he discovered, that 

 instead of the machine containing 300 or 400 yard) of 

 covered copper wire, there were only a foxv inches con- 

 polished brass binding-screws; the re- 

 main apparatus being what is technically called 

 a "dummy." 



As we cannot properly recommend any particular form 

 of microscope, or the production of individual makers, 

 consistently with fairness to all wo have, therefore, an 

 equal dilhYiilty in presenting an illustration of one of the 

 most approved kind. 



That represented in the annexed engraving will, how- 

 ever, give a general idea of the most essential parts of 

 compound microscopes, and will also assist the tyro in 

 putting any of the ordinary class of instruments together. 

 In almost every instance the essential parts are similarly 

 constructed by the different makers, the improvements 

 or special characteristics being generally connected with 

 comparatively minor details of the instrument. 



In Fig. 48 wo have an illustration of an excellent form 

 of compound microscope. 



Fig. 48. 



a, the ftanrt ; & b, two supporters on which the microscope rests ; e, a peg 

 fitting into the frame ; it holds a lens, a, which is used to cast liitlit im 

 opaque object! ; d e, the staire on which the object is placed ; /. 

 ror which throws light through transparent ohjecU; g, a rack-work for 

 niung- and lowering the tube I; h h, two thumb-screws for adjusting 

 the focus ; t, a branch on which the tube / is fixed ; /, the brass tube 

 holding the eye-piece, e. The other parts here represented will be 

 more fully described a> we proceed. 



We may now refer to certain modifications of detail, 

 which cause some instalments to be highly valued l>v 

 microscopists ; and for this purpose we commence witli 

 the lower part of the microscope. The use of the mirror, 

 (, is to illuminate objects placed on the stage, rf e. In 

 this stage is a circular hole, through which the light 



: i-om / passes. Of course the reflector i 

 ployed only for transparent objects. During day-time, 

 the mirror should be so placed as to reflect as much li^ht 

 as possible to the stage, and this is best effected by 

 exposing the reflector to daylight at a window. At 

 night-time, an ar^and, oil, or gas-lamp may bo employed, 

 the light of which should be shaded by means of a chim- 

 ney-glass, of a very light-blue colour. This corrects the 

 yellow colour of artificial light, and so renders it whiter 

 than it otherwise would be. The mirror is so fixed to 

 the arm which supports it as to allow of its being placed 

 at any favourable angle with the stage, and thus to secure 

 at nil times sufficient reflected light. 



A very ingenious maffmunrt has been proposed, in 

 which a piece of pure \\\ ramie 



ware is employed as a reflector. Sucn an arrangement 



prevents that unpleasant glare which light reflected 

 from a polished surface generally has. The ordinary 

 reflector may easily be converted into one of these by 

 covering the surface of the glass with any white pew 

 of which, perhaps, sulphate of lime (plaster of Paris) is 

 the best 



The stage, d, on wliich the object rests, is of con- 

 siderable importance in the microscope. It is gene- 

 rally made in two parts, which easily and yet firmly slide 

 on each other. By moans of these combined motions, 

 the objects placed thereon can be readily shifted to ami 

 fro, so as to be in any required position with the object- 

 glass. An accurate and readily shifting stage is of great 

 use when high powers are employed. Its motions are 

 generally guided by means of a small lever, working in a 

 universal joint; various arrangements, however, are 

 employed by different makers. 



On one side of the stage is fixed, by means of a peg, c, 

 a lens, represented at u, in Fig. 48, whose object is to 

 illumine opaque bodies whilst under examination, and 

 for which the mirror would be useless. The focal point 

 of the lens is brought to bear on the object, so as to 

 afford as much light as possible. The student, however, 

 must not employ the direct solar rays for the purpose ; 

 for not only will the light be too powerful, but it is 

 more than probable that the heat so concentrated would 

 immediately destroy the object, if of an organic nature. 

 Next, above the stage, and screwed into the long brass 

 tube, I, is the object-glass, by which the rays passing from 

 the object are first received, and subsequently transmitted, 

 by means of the eye-piece, to the eye of the observer. 

 Several of these, having different focal lengths, are sup- 

 plied with the compound microscope; and, as we have 

 already intimated, it is essential that such should be 

 achromatic. The image formed by the object-glass is 

 viewed by means of the eye-piece, o, of which two or 

 three aro generally provided, each having different mag- 

 nifying powers. By means of the latter, the image 

 afforded by the object-glass is magnified and presented 

 to the eye. 



Arrangements are required for focussing the lenses; 

 or, in other words, to enable them to present the best 

 possible imago to the eye of the observer, by regulating 

 their mutual distance. This is generally done by moans 

 of two sets of screws and a rack-work motion ; the 1 

 of which is represented at g in Fig. 48, the thumb screw 

 being seen at h. This is what we may call the coarse 

 adjustment, which is still further improved by means of 

 a similar but much finer arrangement represented at p. 

 On the exactness of adjustment of the lenses, and their 

 distance from the object under examination, depends the 

 excellence of the imago as seen by the observer. 



Objects may be mounted, if intended to be preserved, 

 under tliin squares of glass, secured at their edges by 

 moans of pasted paper. The mode of doing tlr 

 described under the head of "Oxy-hydrogen Microscope," 

 <tc. Many objects may, however, be temporarily pi 

 in the microscope by means of pincers or forceps, 

 speotivelj illustrated at v and w. It is often in 

 sary to examine water insects in a living state, and for 

 this purpose a trough containing the liquid must be 

 used; of this kind one is represented at x. 



\Ve have thus endeavoured to illustrate, for the use of 

 the student and our readers generally, the chief parts of 

 modem compound microscopes. Some excellent works 

 have been published on the subject, in which all d. 

 are fully treated; and, perhaps, the best that w. 

 recommend for those of our readers who are disposed 1.0 

 fully into microscopic investigation, is that by Dr. 

 Carpenter, entitled The Microscope and its Revelaf 

 in which the subject is almost exhausted. Other vain- 



; ivatises, of a more elementary character, RII, 

 ll<ilj'-li"tn-s n-itk ilw Microscope, and similar productions, 

 form excellent hand-books to the student. 



We may here describe a very simple form of micro- 

 scope, which can be frequently employed in the stead of 

 larger instruments ; and it can be purchased of most 

 I instrument-makers for a moderate cost. 



Fig. 49 represents one of these. Its princip.il parts 



