MICROSCOPE. 



343 



that a considerable general advantage is lience 

 derived. We can regard no microscope as 

 complete, without an eye-piece of this kind, 

 with a set of ordinary objectives of low powers; 

 for it will certainly do what no other combi- 

 nation with which we are acquainted can effect. 

 The great improvements recently made in the 

 construction of achromatic objectives, and the 

 unquestionable fact that, for exhibiting the 

 minute details of objects, they are infinitely 

 superior to all other kinds, have had, we think, 

 a tendency to blind microscopists to the ad- 

 vantages afforded by other combinations, where 

 it is desired to obtain a view of the general 

 arrangement of the parts of a large object, 

 rather than to investigate its minutiae. 



We should recommend, therefore, that every 

 achromatic microscope should be fitted with at 

 least two Iluyghenian eye-pieces, adapted ex- 

 pressly to the achromatic objectives; and that 

 it should also have a meniscus eye-piece, with 

 a set of ordinary object-glasses of long focus. 

 And the best substitute for such a microscope, 

 at least for the purposes of anatomical or phy- 

 siological research, we believe will be found 

 in Mr. Holland's doublet microscope, which 

 should be furnished with his eye-piece for 

 doublets already described, and with a me- 

 niscus eye-piece and ordinary object-glasses of 

 low power. In this last form of compound 

 microscope, there is the further advantage, 

 that the high magnifying power of the doublets 

 and triplets employed as objectives renders 

 them available as simple microscopes ; and 

 this cannot be said of achromatic object-glasses, 

 which have not yet been usually made of 

 shorter focus than one-tenth of an inch, and 

 which are not, therefore, of much use in them- 

 selves. No one, however, can be regarded as 

 entitled to form positive conclusions in regard 

 to difficult questions of microscopic enquiry, 

 until he has availed himself of the very best 

 means of observation at his command ; and 

 these are certainly to be found only in achro- 

 matic microscopes of the highest class. 



For viewing; large opaque objects, achromatic 

 objectives of low power are often very useful, 

 on account of the large quantity of light they 

 admit, which supersedes the necessity of arti- 

 ficial illumination ; this is a particular advan- 

 tage in anatomical investigations, in which it 

 is often especially necessary to avoid the re- 

 flection of condensed light from the surface of 

 the object, on account of the confusion which 

 is thereby occasioned. 



The achromatic objectives at present usually 

 made on the continent consist of seta of three 

 or more, of which one, two, or three may be 

 used at once. In this manner considerable 

 variety of power may be gained; but the 

 highest degree of perfection in the performance 

 must be sacrificed to obtain it, since no single 

 objective consisting of two lenses only can be 

 thoroughly corrected, and each combination 

 ought to be corrected for itself alone. The 

 best achromatics made by British artists consist 

 of combinations of two or three compound 

 lenses, which cannot be separated; and thus 



Fig. 165. 



Section of the English achromatic combination. 



every required power must be furnished by a 

 distinct combination. The expense of a mi- 

 croscope fitted with the requisite number of 

 these, however, is a great bar to its general 

 employment. Other combinations have been 

 constructed, therefore, in which the lens next 

 the object may be removed, so as to diminish 

 the magnifying power considerably ; and the 

 corrections are so adjusted as to be nearly the 

 same when the two or when three compound 

 lenses are used together. The difference be- 

 tween the performance of the best of these, 

 and that of those most perfectly adjusted, is 

 not, for general purposes, of much importance. 

 Two sets of these separating lenses, a high 

 and a low one, giving four powers, therefore, 

 which may range from an inch and a half to 

 one-eighth of an inch focus, will adapt the 

 microscope, with the eye-pieces we have men- 

 tioned, to a great variety of purposes. 



The power may be further varied by length- 

 ening the body of the microscope, by drawing 

 out the eye-piece, which should always be 

 made capable of this kind of movement. This 

 operates by increasing the distance from the 

 object-glass of the image formed by it, and 

 therefore augmenting the size of the image; 

 the object must of course be brought some- 

 what nearer on the other side. We have al- 

 ready stated that the length of the body cannot 

 be much increased with advantage ; but a mo- 

 derate variation will be found useful in many 

 ways. It enables the magnifying power to be 

 adjusted to almost any point intermediate 

 between those given by the different objectives. 

 Thus, one may give a power of 80 diameters, 

 and another a power of 120 ; by using the first, 

 and drawing out the eye-piece, the power may 

 be increased to 100. Attain, it is often very 

 useful to make the object fill up the whole, or 

 nearly the whole, of the field of view. This is 

 especially the case, when it is itself not very 

 transparent, and requires a strong light to 

 render its details visible ; in which condition 

 a glare entering around its edges would very 

 much interfere with its distinctness. When 

 opaque objects, also, are being viewed by con- 

 den^-ed light, in the modes hereafter to be 

 stated, it is often extremely desirable to make 

 them, or the discs on which they are mounted, 

 fill up the whole field. In cither case the 



