MICROSCOPE. 



G57 



practicable arises from the fact, that the chromatic 

 aberration of lenses is less perceptible by it than 

 by artificial light. By day-light, however, is not 

 to be understood the direct light of the sun, which, 

 except in the case of the solar microscope, is the 

 very worst illumination that can be employed ; for 

 it causes such a mist and confusing glare in the 

 field, that no object, transparent or opaque, can be 

 seen by it with due distinctness. It is necessary 

 also to point out the kind of artificial light which 

 is most suitable for microscopic purposes ; for ob- 

 servation is frequently defeated and much illusion 

 produced by a bad illumination. The light of a 

 tallow candle, even of a moulded one, is very ob- 

 jectionable ; the flame is almost continually in 

 agitation, and the flickerings cause an unpleasant 

 motion in the field, that destroys all the defi- 

 nition of the microscope, however perfect it may 

 be. An argand lamp has been much recommended 

 for its steady and intense light ; and if the observer 

 be provided with an instrument having all the ne- 

 cessary apparatus of stops, lenses, and mirrors for 

 its management, it is perhaps the most efficient 

 light of an ordinary nature that can be obtained. 

 We have ourselves seldom used any other light 

 than that of a wax candle, which, if it be not dis- 

 turbed by a current of air, gives an uniformly steady 

 flame, and produces a fine motionless illuminated 

 field, well adapted for transparent objects. This 

 light, it is true, is not so intense as that of the 

 argand lamp; but we find this an advantage 

 rather than otherwise, so long as we have at hand 

 the necessary means for condensing and directing 

 it. 



The quantity of light admitted or transmitted 

 must be regulated by the aperture of the lens and 

 the nature of the object under observation, since 

 too much light is as objectionable as too little. 

 Simple lenses of low power applied either to 

 opaque or transparent objects, do not require a 

 strong light ; for they admit a much larger pencil 

 of rays than the deepest magnifiers, and, by conse- 

 quence, if used with too intense an illumination, 

 they both injure the eye and exhibit the object 

 under a perplexing brilliancy. As the powers in- 

 crease in depth, the light must increase in inten- 

 sity ; for in ordinary magnifiers the aperture through 

 which the rays are to pass, continually contracts as 

 we ascend to the most powerful amplifiers ; and it 

 must be evident that a limited aperture requires a 

 concentration of light that will compensate for the 

 smallness of the pencil admitted through it. A 

 diamond lens will not need so much light as a glass 

 one of the same magnifying power, and for this 

 obvious reason, the diamond presents a larger 

 surface and takes in a much greater number of rays. 

 The same remark applies in a less degree to the 

 other jewel lenses. Doublets and Triplets are 

 expressly directed to the examination of the 

 minutest and most delicate objects that can be sub- 

 mitted to the microscope ; and consequently the 

 due management of the light with reference to 

 them is most important, as the slightest deficiency 

 or excess of illumination is sufficient to perplex the 

 observer. We are not able to give definite instruc- 

 tions on this point, as every observation requires a 

 peculiar adaptation and adjustment; as a general 

 rule we advise that in every case the effect of both 

 oblique and direct rays of different intensity be 

 tried. Indeed no directions can be given on the 

 management of light and illumination that will not 

 require to be modified according to circumstances. 



To say no more, the variety of the human eye in 

 different individuals would nullify any definite rule 

 that could be laid down. The observer must ac- 

 quire by practice, and from such general observa- 

 tions as we have offered, the necessary skill and 

 discretion. 



MAGNIFYING POWERS It is desirable that the 



microscopic cabinet should be provided with a 

 judicious selection of magnifying powers, and the 

 series should have considerable range ; since the 

 objects which nature offers for examination are 

 numerous and almost infinitely varied, and each 

 class requires a peculiar amplifying power for its 

 development. An object may be submitted to too 

 high a power, and exhibit nothing to awaken in- 

 terest or excite attention ; but on applying a lower 

 amplifier, a formation is discovered that charms us 

 by its beauty or astonishes by its wondrous ar- 

 rangement. We look at another object slightly 

 magnified, and it remains an unmeaning atom ; yet 

 no sooner do we deepen the power, than obscurity 

 is dispelled, and new information dawns upon us ; the 

 small material particle whose presence the unassisted 

 eye could not detect, exhibits a manifestation of 

 creative power, in the contemplation of which all 

 the faculties of the mind are absorbed. Mr Prit- 

 chard distinguishes, and with his usual acumen, be- 

 tween the penetrating and the defining power of 

 magnifiers ; the first of these he refers to aperture, 

 or, in other words, to the quantity of rays gathered 

 up, and the latter to amplifiers, whose chromatic 

 and spherical aberrations are under due correction. 

 The penetrating power is necessary to develope 

 the structure of the scales on the wings of butter- 

 flies, &c., and the defining power is required for such 

 objects as the hair of a mouse. 



The following is a table of progressive magni- 

 fiers, whose foci range from one inch to one-thirtieth 

 of an inch, and opposite each lens is placed its 

 focus, and the magnifying power on the line, surface, 

 and cube, assuming distinct unaided vision to be 

 effected at ten inches. These are the amplifiers we 

 have used in our own observations, and by which 

 our illustrative drawings were constructed. We 

 venture to recommend them as a perfectly satisfac- 

 tory series for general purposes. 



F L s c 



100 1000") These may 



400 8000 f be used with 



1600 64000 fa compound 



6400 512,000j body. 

 10,000 1,000,000 



1089+ 35,937+") Incrcaro of 

 inn nnn ( penetratm* 



1 st Magnifier 

 2nd 

 3rd 

 4th 

 5th 



Coddington Lens 

 Wollast. Doub. 

 Ditto 



8,000,000 UJ 



27,000,000 



nd defining 



It is to be observed with respect to the foci of 

 double convex lenses that they always measure 

 from the centre of the medium and not from the 

 apex of the surface exposed to the object; for the 

 magnifying power that is calculated from a focal 

 distance between the object and proximate sur- 

 face will in some cases be widely wrong, as may 

 be seen in the preceding table, where the Codding- 

 ton lens of one-tenth inch focus is found to have only 

 one-third the power of an ordinary double convex 

 lensof similar focus. The thickness of double convex 

 lenses does not usually bear any considerable pro- 

 portion to their focal distances; and hence it pro- 

 duces no important error in a practical view, to 

 take the distance between the object and proxi- 

 mate surface as the true focus. In the case of a 

 spherule, sphere, or hemisphere, however, it is 

 obvious that the thickness of the medium is of im 

 2x 



