ON THE USE OF THE MICROSCOPE. 577 



selves involve this. The million-fold powers blazoned forth by quackery are 

 nothing more than the most absurd statements of cubical enlargement, and are 

 calculated by the distance of the lens from the surface which receives the image, 

 in the same way as in the magic-lantern, and by which all accuracy of defini- 

 tion the very thing requisite in scientific researches is lost. 



As a matter of course, we may mention that concave mirrors may be employed 

 instead of transparent lenses, in the same way as in the telescope : this was first 

 done by Amici, of Modena, and at a time when the aplanatic lens was not dis- 

 covered it certainly was a very desirable improvement. This arrangement, how- 

 ever, has almost entirely lost its value at the present time, for, independently 

 of the difficulty of always keeping the glass or mirror perfectly clean, it can 

 only be made use of to afford a very low magnifying power, as otherwise the 

 object could not be placed, and thus the greater portion of the magnifying power 

 has always to be performed by the eye-piece, and which is liable to errors of 

 spherical aberration in a much higher degree than is the case with the dioptrical 

 instrument. 



It is evident, from the representation we have just given, that the excellence of 

 the microscope depends principally on the good quality of the lenses, and, in the 

 compound microscope especially, on the correctness and definition of the object- 

 glass, since every error belonging to the image is still more magnified through the 

 eye-piece. There were two errors particularly which have been only recently 

 remedied, but that with great success, namely, the chromatic and the spherical 

 aberration, which are now removed, the former by achromatic lenses, and the 

 latter, in simple microscopes, by Wollaston's or Chevaliers doublets ; in the com- 

 pound microscopes by aplanatic object-glasses. The instrument also, in which 

 the eye-piece removes the spherical aberration by means of aplanatism, is very 

 excellent. I do not think that much more can be seen by any microscope now 

 manufactured in Europe, than by the combination of the three strongest object- 

 glasses with the aplanatic eye-piece of Pldssl's microscope, although it only gives 

 a two-hundred linear magnifying power. The dimensions in the stronger powers 

 of the same artist, in which the aplanatic eye-piece is not used, are certainly 

 more considerable, but we do not distinguish more points or lines in the image, 

 and therefore we do not see more, but only rather more conveniently. It 

 follows, from the preceding explanations, that we should only use instruments 

 furnished with achromatic doublets in the simple microscopes, and in compound 

 microscopes furnished with achromatic, or at least with aplanatic, object-glasses, 

 in order to obtain results as much as possible free from optical errors. Schiek 

 in Berlin, and Plossl in Vienna, undoubtedly furnish the best instruments at the 

 present time. Plossl's instruments are pretty nearly on an equality with Schiek's 

 in all combinations in which the strongest object-glass does not occur. On the 

 other hand, all combinations with the three strongest object-glasses are certainly 

 to be preferred, and form the best instrument that has as yet come under my 

 observation. The brass work is undoubtedly better in Schiek's instruments. 

 Next to the instruments of those distinguished artists, we may probably name 

 the more recent instruments of Pistor and Hirschmann in Berlin, of Oberhauser 

 and Chevalier in Paris ; of the latter I have certainly not seen any, but infer as 

 much from the results obtained through them by the French. The more recent 

 English instruments seem to be so much inferior to those we have mentioned, 

 that they bear no comparison. I likewise have not seen any of these, but there 

 certainly is no lack of clever observers in England ; and since, therefore, no 

 microscopical botanical researches of any importance have been very recently 

 furnished by England, excepting those of Robert Brown, and since what obser- 

 vations the English have made can frequently be easily refuted by a cursory 

 glance through our glasses, this deficiency can only be attributed to the de- 

 fectiveness of their instruments.* 



* I should not, I think, be doing justice to my countrymen were I to allow the 

 above remarks to pass without comment. Since the time the above was written, many 

 improvements have taken place in the construction of the microscope, and in no coun- 

 try in Europe have so many of these been made as in England. Even at the time the 



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