342 



MICROSCOPE. 



referred to, whom we believe to have had great 

 practical experience in the matter, " for pro- 

 ducing these results. Close study of the for- 

 mula; for achromatism given by celebrated ma- 

 thematicians will do much ; but the principles 

 must be brought to the test of repeated expe- 

 riment. Nor will the experiments be worth 

 any thing, unless the curves be most accu- 

 rately measured and worked, and the lenses 

 centred and adjusted with a degree of preci- 

 sion which, to those who are familiar only with 

 telescopes, will be quite unprecedented." We 

 are not favourable to the use of a very high 

 magnifying power in the eye-piece; and we 

 believe that it will be discontinued in propor- 

 tion to the perfection attained in object-glasses. 

 We have seen microscopes of foreign construc- 

 tion in which only object-glasses of compara- 

 tively long focus were employed, and the re- 

 quired power was made up by the great con- 

 vexity in the eye-glass ; but the performance of 

 these was not to be compared to that of instru- 

 ments of British manufacture. Our own expe- 

 rience leads us to think that there are very few 

 objects of which more can be made out with a 

 deep than with a shallow eye-piece, the dimi- 

 nution in distinctness and loss of sight being 

 nearly sufficient to counterbalance the gain 

 derived from increased power. Hence the mag- 

 nifying powtr of an instrument is by no means 

 to be regarded as an indication of its excel- 

 lence ; for that is to be considered as the best 

 which, ccteris puribus, will show the most 

 with the lowest power. It may be scarcely an 

 exaggeration to affirm that there are few objects 

 of which the details may not be as well made 

 out by an achromatic microscope magnifying 

 but 100 diameters, as by the best ordinary 

 microscope magnifying 1000 ; and there are 

 many objects shown with the greatest readi- 

 ness by the former, which are totally inscru- 

 table by the latter. 



Next in order of optical perfection to the 

 achromatic microscope with the Huvghenian 

 eye-piece, we are disposed to rank the doublet 

 microscope, invented by Mr. Holland. This 

 gentleman has proposed to adapt to his dou- 

 blets and triplets a compound body, con- 

 taining an eye-piece somewhat resembling the 

 Huyghenian, but differing from it in having the 

 lenses fixed at a distance equal to the whole 

 sum of their foci. " By this increase of dis- 

 tance, light and defining power are gained, 

 although the magnifying power and the field 

 of view are diminished ; but at the same time 

 the latter is rendered very perfect." Having 

 ourselves had a microscope constructed upon 

 this principle, we can speak in very high terms 

 of its performance. The field of view will 

 appear very small to those accustomed to the 

 use of eye-pieces of high power ; but every 

 part of it is brilliantly illuminated, and diffi- 

 cult test-objects are exhibited bv it with a 

 sharpness and definition which we have seldom 

 seen equalled. For objects which require to 

 have a large surface in view at once, such a 

 microscope is inappropriate ; but for the pur- 

 pose of minute examination of those in which 



the parts may be studied independently, we 

 regard it as the best substitute for the achro- 

 matic microscope ; and we can strongly re- 

 commend it to those who are debarred by the 

 price of the latter instrument from possessing 

 themselves of it. In employing doublets or 

 triplets of high power as objectives in such 

 a microscope, care must of course be taken 

 (as when they are used singly) to avoid injuring 

 them by contact with the object. For the most 

 difficult test-objects, a triplet of ^th of an inch 

 focus should be employed ; and the powers 

 should successively diminish down to a dou- 

 blet of |th of an inch, which may be advanta- 

 geously employed for opaque objects. It is 

 necessary to state that the performance of this 

 microscope will very much depend upon the 

 attention paid to the illumination of the 

 object; on this we shall hereafter treat in 

 detail. 



We shall next speak of eye-pieces which 

 are intended to increase the size of the field in 

 the most advantageous manner, without regard 

 to the perfection of minute details. This ob- 

 ject is ordinarily attempted by the substitution 

 of two plano-convex lenses, or double-convex 

 lenses of low curvature, for the single lens of 

 the eye-glass. It has been proposed to make 

 the same change in the field-glass ; but, in our 

 opinion, the loss by reflexion from two addi- 

 tional surfaces is by no means compensated by 

 the diminution of aberration. We are our- 

 selves, however, in the habit of employing an 

 eye-piece, which we regard as greatly superior 

 to that in ordinary use. It consists of a me- 

 niscus having the concave side next the eye, 

 and a convex lens having the form of least 

 aberration, with its flattest side next the ob- 

 ject, nearly resembling, therefore, Herschel's 

 aplanatic doublet. The field-glass is a double- 

 convex lens of the form of least aberration. 

 With this eye-piece we are enabled to obtain a 

 field of 14 inches diameter (measured at the 

 usual distance 10 inches) equally distinct and 

 well illuminated over every part, and admi- 

 rably adapted for the display of sections of 

 wood, wings of insects, and objects of a simi- 

 lar description, and also for opaque objects. 

 When employing it for these purposes, we 

 much prefer the use of ordinary double-convex 

 objectives to achromatic lenses ; for the latter, 

 being adjusted for a much smaller field, pro- 

 duce an image which is only distinct in the 

 centre ; and the former, being of low power, 

 may have an aperture quite sufficient to admit 

 the requisite amount of light. Even with 

 deeper objectives, the performance of this eye- 

 piece approaches much more closely to the 

 effect of an achromatic microscope, than would 

 be supposed by those who have only seen the 

 ordinary one; and, when made on a small 

 scale, it may be advantageously substituted for 

 Mr. Holland's for all but the most difficult 

 objects. The two additional surfaces are of 

 course disadvantageous, by reflecting some of 

 the fainter rays proceeding from the more 

 delicate markings; but the increased magnify- 

 ing power is gamed with so little aberration, 



