THE STAKHOI'E LEXS. J 



UNDULATORY FORCES. LI G HT. 



05 



ment of the oxy-hydrogen microscope, by which a large 

 number of spectators are enabled to see the magnified 

 im:ige of ail object simultaneously. With our remarks 

 on these instruments, we propose giving instructions to 

 the student, which will assist him in collecting, preparing, 

 and mounting objects of all sorts ; and thus we hope to 

 make our subject complete and useful to all our readers. 

 Perhaps the simplest microscopic arrangement we can 

 mention, is that illustrated in the annexed engraving, 

 which represents a piece of window-glass, under whose 

 lower surface a drop of water or oil depends. The drop, 

 of course, assumes a spherical form, and thus produces a 

 plano-convex lens. In cases where no other kind of 

 microscope can be procured, this is at mice a ready and 

 Fig. 45. tolerably efficient in- 



strument. We need 

 scarcely remind the 

 reader who has pe- 

 rused our previous remarks on lenses,* that almost any 

 spherical and transparent substance will, as it forms a lens, 

 answer to some extent for microscopic purposes. Even 

 globules of melted glass may be thus employed. For 

 fine comp mud instruments, lenses of precious stones, such 

 ai the diamond, have been applied. These are valuable 

 on account of their purity, homogeneity of structure, 

 and high powers of refr.ictiou ; but through their ex- 

 treme hardness they are difficult to polish, and hence are 

 'lingly expensive. Rock crystal, and various kinds 

 uf 'paste," or glass used in making artificial gems, have 

 been also made into lenses with varied success. The 

 belt kind of glass is almost always, however, as a nil-, 

 used in the manufacture of lenses for microscopic 



In all ordinary spherical lenses there exists an objec- 

 tion to their utility, owing to the aberration of light 

 wlii h is a necessary consequent-*) of their form. We 

 have already explained the reason of this ;t and to 

 remedy it, and other evils, two very useful, simple 

 nii.-r .scopes are constructed ; namely, the Stanhope and 

 Coddington l.'iises. 



Tho Stanhope microscope consists of one lens, whose 

 two curved surfaces are so ground as to liave different 

 Joci, and the focal point of each terminates near the 

 surface of the other. Hence, in using the arrangement, 

 the object to be examined is placed on the surface of 

 one of tin- cnd-i of tlie lens, and is viewed through the 

 other. This is a most useful and complete contrivance ; 

 and, in the absence of more complicated instrument*, 

 will be found of great value to the student. It is of 

 small size, and but of slight cost ; and as a field com- 

 panion to the naturalist, it is of considerable advantage. 

 Fig. 46 represents a Stanhope lens of the ordinary form, 

 Fig . 46. and is about half the size of the 



instrument itself. 



The Coddington lens is an equally 

 valuable little microscope ; and its 

 peculiarity consists in the fact, that 

 a portion of the lens between its two 

 curved surfaces is removed by grind- 

 ing, which prevents many of the 

 annoyances occasioned by spherical aberration. This 

 makes the instrument of a much more complete kind 

 :h.m the simple lenses to which we at first referred. 

 Fig. 47 represents a Coddington lens of about half 



Fig. 47. 



the usual size, as sold by the 

 instrument-makers. 



Either of the above instru- 

 tr~n r^ inents may be purchased for 

 " two or three shillings ; and as 

 some of those who peruse our 

 page* may not have the means 



to procure instruments of a high class, we can re- 

 commend no bettor microscopic arrangements for or- 

 dinary purposes. By means of the Stanhope lens the 

 nt will IM- enabli-d to examine a ; ;y of 



i>uautifiil objects, both of an animal and vegetable nature. 

 The structure of flowers and leaves, the wings, down, 

 ind other parts of insects, may bo easily studied thereby. 



Sec axtt, p. 48, rt Kf . f Anlr ]>. 1 1. 



'.. r. 



We know of few more interesting amusements than a 

 morning spent in the microscopic investigation of the 

 infinite variety of objects which the bountiful hand of 

 Nature spreads in all directions about us. Whilst wo may 

 view, with awe and admiration, those immense results 

 of the Creator's power which the telescope reveals to us, wo 

 can, by means of the sister instrument, the microscope, 

 dive into the minuti;e of Creation, and reap an immense 

 amount of intellectual pleasure therefrom. 



Having drawn attention to some of the best kinds of 

 simple microscopes, we shall now pass on to consider the 

 construction and uses of those called "compound," in 

 which an extensive power of examining the most minute 

 objects is afforded by means of a combination of lenses, 

 and other apparatus. 



COMPOUND MICROSCOPES. 



FROM the remarks which we have made in our previous 

 pages, when speaking of lenses, the reader will have 

 observed that the magnifying power of a lens is inversely 

 as its focal length ; or, more plainly, the shorter the 

 focal length, the greater is the magnifying power of tho 

 lens employed. It would follow, therefore, theoretically, 

 that a single lens may be manufactured which would 

 afford any magnifying power which might be desired. I n 

 practice, however, this is impossible; because, when tho 

 focal length of a lens i.s exceedingly small, the eye, lens, 

 and object would require to be so near each other for the 

 purposes of microscopic investigation, as to entirely pre- 

 vent every chance of the observer using such an arrange- 

 ment. 



Lenses of almost the shortest possible focal length may, 

 however, readily be employed, when combined with an- 

 other lens which magnifies the image produced by the 

 former. Such is the combination found iu the compound 

 microscope, in wliich the object-ylasa, or lens of short focal 

 lengt: , the rays passing from the object under 



n, and produces an image thereof ; such imago 

 is vieiM-d by means of another lens of far less magnifying 

 power, and therefore of greater focal length, which, being 

 next the eye of the observer, is called the eye-ylusf. 



By such an arrangement an extended field of view is 

 obtained, and tlie magnifying power which may bo em- 

 ployed is only limited, practically, by tho skill of tU" in- 

 strument-maker. 



We must, however, remind the reader, that if ordinary 

 glass lenses are employed, all the effects of chromatic anil 

 spherical aberration (of which we have already spoken 

 so frequently) are called into play ; and hence all tho 

 rays except those passing nearest to the centre of tho 

 lenses of the instrument must be cut oil' by means of 

 stops, which of course would cause a great loss of light. 

 This difficulty has, however, been remedied in another 

 way, by using achromatic lenses; and such instruments 

 as the compound achromatic microscopes, manufactured 

 by Ross, Smith and Beck, and other eminent makers, 

 are perhaps amongst the most perfect productions of 

 human skill and ingenuity. 



Jt, of course, will be out of our power to recommend 

 tlie best form of microscope for the student, varying as 

 they do in so many particulars. As, however, the means 

 of many of our readers may not permit the purchose of a 

 high-priced instrument, we may state, that for ordinary 

 purposes, a good microscope, containing most of the 

 modern improvements, may be obtained for from three 

 to five pounds, from the maker. And sometimes, at 

 sales by auction, a very good instrument may be purchased 

 at a comparatively low cost. In such cases, however, the 

 purchaser should not trust to his own judgment, if in- 

 experienced, but should first have the arrangement 

 examined by an optician, so as to prevent disappoint- 

 ment. We may stite, that in nearly all our large towns, 

 philosophical instruments are made expressly for public 

 sales, at which both the article and the purchaser are 

 frequently "sold" together. As an instance of this, we 

 may mention the case of a friend who purchased, in one 

 of the largo towns in Scotland, what he thought to be a 

 lirst-rato galvanic coil machine. On attempting to obtain 



