Class X.] MICROSCOPES—RECENT IMPROVEMENTS OF THESE INSTRUMENTS.
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joint focus; a principle of compensation originatingtheoretically, we believe, with the late Mr. Rogers, ofLeith, and carried into practice with much success byPlossal. This instrument was found to give very perfectimages, with no uncorrected colour, this, together withits being the only telescope of the kind exhibited, andthe construction deserving of encouragement, has inducedthe Jury to consider it worthy of a Prize Medal. It ismounted on a stand, which, though defective in solidity,may be used as an equatorial mounting in any latitude.
Busch (Prussia, No. 80, p. 1053) exhibits an achro-matic telescope of 5 feet focal length and 48 lines aper-ture. On examination, this telescope was found to bepretty good. The same exhibitor has fourteen other tele-scopes of a small size. Honourable Mention is awardedto Mr. Busch.
Microscopes.
The Exhibition is rich in its collection of microscopesof all kinds, comprising instruments varying from thesimplest forms to the most elaborate. Many of the latterhave never been surpassed for power, goodness of object-glass, definition, large angular aperture, beauty of work-manship, great convenience of the subsidiary parts, com-bined with great permanence of adjustment.
The microscope lias been rendered second in import-ance only to the telescope by its application to physicalresearches. To the science of geology it has been madeto display its great powers in the discovery of manystrata of considerable thickness, almost entirely com-posed of infusorial remains, too small for the naked eyeto distinguish or appreciate their exquisite beauty ofform and structure. To members of the medical profes-sion its services are indispensable; and in various waysit may become of essential use to every class of society.By knowing the microscopic appearance of different arti-cles, a microscopist may, with ease, detect adulterationin any form, as in adulterated bread or flour for instance,the presence of any grain other than that of wheat willbe readily discovered, the starch in each variety of grainbeing possessed of a distinctive character, and in textilefabrics the intermixture of cotton with linen can be im-mediately detected. But these are two isolated illustra-tions out of many far too numerous to be mentioned.
When Tully constructed the first achromatic object-glass in this country, in the year 1824, Dr. Goring said,“ that microscopes were now placed on a level with tele-scopes, and, like them, must remain stationary in theirconstruction.”* This prediction fortunately has notbeen verified, as the most careful examination of theinstruments in the Exhibition has fully exemplified. Totrace the causes of the steady and progressive improve-ments which have resulted in the production of micro-scopes of such perfection as those exhibited is mostdesirable. Mr. Bowerbank, who has always taken anactive part in these improvements, lias kindly furnishedthe necessary information.
About the year 1824 the first effective movement wasmade towards applying achromatic object-glasses to thecompound microscope. In the same year the report ofSelligue’s microscopes was made to the Royal Academyof Science ; and Amici resumed the subject after aninterval of nine years. The late Mr. William Tully pro-duced. on March 1st, 1824, at the instigation of the lateDr. Goring, his triple glass, J-inch focal length, it beingthe first achromatic combination for the microscopemade in this country. But in the following year, on Mr.Lister showing him the disadvantageous figure of thisconstruction, he altered it to his well-known nine-tenths.This glass had an aperture of 18 degrees, and workedwell with eye-pieces to about 120 linear. Subsequentlya second smaller triplet was placed before the nine-tenthsglass, and a power was obtained, with beautiful defini-tion, of 300 linear, with a pencil of light of 38 degrees.The glasses thus combined were about equal to a lens ofthree-eighths of an inch focus.f
* Exordium to “ Microscopic Illustration,” 1820.
t See “Quarterly Journal of Sciences” for 1825, No. 37,p. 132; also for 1827, No. 44, p. 2G5. Specimens of these
With a happy combination of mathematical knowledgeand practical experience, Mr. Lister continued to pur-sue the subject, and in the year 1S30 he published inthe “ Philosophical Transactions ” his paper on ‘ Apla-natic foci of the double achromatic object-glass, withother properties belonging to it, and a means derivedfrom them of correcting by combination both sphericaland chromatic aberration and coma.’ Upon this com-munication has been based the whole of the progressiveimprovements to their present state of perfection. Al-though the author of this paper has not since publishedthe further fruits of Ills labours, lie has continued hisexertions, and the results have been freely communicatedto those celebrated makers among our own countrymen,who have aided and assisted him by their abilities inobtaining the beautiful achromatic combinations, whichare now so abundant among our ablest anatomists andnaturalists.
Mr. William Tully, who led the way in the manufac-ture of achromatic combinations, died about the close ofthe year 1835. At this time Messrs, lioss and Powellhad taken up the manufacture, but without much ad-vance on the first simple application of the principleuntil the beginning of 1S37, when, at the suggestion ofMr. Lister, Mr. Ross constructed a differently-arrangedcombination, but on the same basis. It consisted of twodouble and one triple compound lens. This combinationwas successfully worked out, and was designed to pro-duce a great increase of defining power; and it was atthis time that the thought occurred to Mr. Ross to leavethe front lens moveable, so that by varying its distancefrom the others, the glass might be adjusted either forcovered or uncovered objects.
In the summer of the same year a new construction,composed of two achromatic lenses for a lower power,was suggested by Mr. Lister, and executed by Mr. Ross.In 1843, Mr. Ross constructed a combination of two trip-lets and one doublet, by means of which an increasedangle of aperture and improved definition were obtained.One or the other of the above combinations are now em-ployed by our three chief makers, Messrs. Powell andLealaiul, Ross, and Messrs. Smith and Beck, to whomMr. Lister also communicated the results of his expe-rience in the construction of achromatic combination, eachoccasionally surpassing the others in the quality of theglasses; and their successive exertions have resulted inthat superiority of the numerous and beautiful combina-tions which they have produced over all others in theExhibition.
To Sir David Brewster wc are indebted for manyvaluable suggestions for the improvement of the instru-ment, especially for the best method of illuminating bytransmitted light, by the application beneath the stage ofachromatic combinations, in place either of simple reflec-tion of light from plane or concave mirrors, or of itsconcentration by means of the Wollaston condenser. Bythis method of illumination, minute and delicate objectsduring examination are presented to the eye under themost favourable circumstances, and a beauty and correct-ness of definition obtained which no other mode ofillumination is capable cf producing.
To the same source we are likewise indebted for thevaluable application to the microscope of the apparatusfor the polarization of light, which has so powerfullyassisted us in the investigation of delicate and transparentanimal and vegetable tissues.
In conclusion, it may be observed that the low-pricedinstruments exhibited by Messrs. Ross, and Smith andBeck, are deserving of high commendation. The brass-work is good in principle, steady and free from tremor inoperation: and the powers, varying from 1-inch to aquarter-inch focus, inclusive, are by far the most gene-rally useful in the whole range of microscopic combina-tions, especially for educational purposes.
It must be remarked that it is advisable that the angleof aperture of the combinations should not be extendedto its utmost possible limit, when destiued for the ge-
combinations are still in the possession of Messrs. Lister,Bowerbank. and Loddiges,