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HARDWICKE'S SCIENCE-GOSSIP. 



[Oct. 1, 1870. 



MICROSCOPIC AMPLIFICATION.* 



fTIHE problem of amplifying the image formed by 

 -L the objective of a microscope beyond the 

 limits which are fixed by the ratio between the con- 

 jugate foci of the objective itself, by means of inter- 

 posed lenses, called amplifiers, without destroying 

 definition, and without distortion, has been the 

 subject of many experiments ; but, thus far, only a 

 very limited enlargement has resulted. 



By increasing the ratio between the conjugate 

 foci of the objective, the size of the image is in- 

 creased; and, in the hope of making this principle 

 available, microscopes of enormous length have 

 been constructed, and so arranged, for convenience, 

 as to return the image by reflection to an eye-piece 

 fixed near the object ; but no objective has yet been 

 made which will preserve definition with a differ- 

 ence between the lengths of its conjugate foci, great 

 enough to give such enlargement to the image as is 

 here shown; and the shortening of its anterior 

 focus rapidly reduces the working distance of the 

 objective to an impracticable length. 



Enlargement may be effected by reducing the 

 focal distance of the eye-glass; but the limits are 

 very narrow within which the convexity of that 

 lens can be increased, without obscuring and 

 distorting the image; and experience inclines 

 microscopists more and more to the use of longer 

 eve-pieces and shorter objectives. 

 * The utmost effect of all former arrangements, so 

 far as results have been published, has been to 

 magnify the object, with good definition, less than 

 fifteen thousand diameters ; beyond which, no re- 

 corded result is known to microscopists in this city, 

 and no such enlargement has been observed here. 



The amplification now exhibited to you reaches 

 9S,000 diameters, and is obtained without removing 

 the object from the eye beyond the distance per- 

 mitted by an ordinary microscope, and without 

 losing definition or sufficient light. The microscope 

 used" is the well-known instrument by Mr. Zent- 

 mavcr, of Philadelphia, furnished with an objective 

 by Mr. William Wales, of this city, having a focal 

 distance equivalent to one-thirtieth of an inch, 

 which is worked in combination with the ordinary 

 " C " eye-piece of Zentmayer, the focal length of 

 whose eye-lens is half an inch. A combination of 

 lenses, which is believed to be new, is interposed 

 between the objective and eye-piece, and is now 

 exhibited to you. The formula of this combination 

 will be submitted on a future occasion. 



The object first subjected to examination is an 

 angulation, prepared by Eourgogne, the distance 

 between the lines of which is one fifty-two thou- 



* A paper read before the Bailey Microscopical Club, New 

 York, May 31st, 1870, by Mr. Edward A. Dickerson, of that 

 city, a Member of the Club. 



sandths of an inch. An apparent field is presented 

 which measures eight and a half inches in diameter 

 at the distance of eleven inches from the eye. 

 Diametrically across the field, only four and a half 

 rows of the hemispheres of the angulation appear : 

 and the entire area contains only fifteen of them. 

 The definition is clear and equally good over the 

 whole field. The hemispheres appear free from chro- 

 matic aberration, and are of a light grey colour on 

 their illuminated surfaces. The crescent-shaped 

 shadows and the interspaces are perfectly distinct, 

 and the general effect is similar to that of a plain 

 disk of light-brown material, eight and a half inches 

 in diameter, placed at eleven inches from the eye, 

 and illuminated by oblique light, upon which fifteen 

 hemispheres of grey plaster have been symmetrically 

 arranged. The accompanying photograph, made 

 from*a model, exhibits the true size and general 

 appearance of the field. 



The Surirella gemma under an enlargement ot 

 ninety-eight thousand diameters, and human volun- 

 tary muscle under twenty thousand, produced by 

 this method, exhibit new and remarkable appear- 

 ances, which will be the subject of future study and 

 exhibition to the club. 



The illumination of the object, when amplified to 

 forty-five thousand diameters, is sufficiently bright, 

 with Dr. Higgins' student's lamp arrangement; but 

 for higher powers more light is required, which is 

 conveniently obtained .by an incandescent calcium 

 pencil the rays from which are made parallel by an 

 achromatic convex leus, and then condensed upon 

 the object by an Abraham's prism, which last-named 

 instrument cannot be too highly praised. A coal-oil 

 lamp, instead of the calcium pencil, gives fair illu- 

 mination under this high power. It is apparent that, 

 thus far we are using this combination under serious 

 disadvantages, and that much more may be expected 

 from it when properly worked. When used m an 

 ordinary house, a loud voice speaking in the room 

 makes the image flit out of view, and a step on the 

 floor destroys all distinctness of vision. It is neces- 

 sary to plant the instrument upon a stone lounda- 

 tion before all its powers can be developed ; because 

 the vibrations of a dwelling-house, whose amplitude 

 may not be enough to be seen in the field, must im- 

 pair the sharpness of outline, just as an unsteady 

 foundation docs with the image in a telescope of 

 large power, even when the tremor cannot be other- 

 wise detected. 



The "fine adjustment" of the Zentmayer is too 

 coarse for this work ; although the objective is 

 moved by it seven hundred times slower than the 

 fino-er moves which touches the milled head ; yet the 

 lightest possible pressure upon the screw changes 

 the focus, although the milled head be not sensibly 

 moved; and when the finger is withdrawn another 

 change occurs, so that very delicate manipulation is 



required to leave the adjustment exactly ri 



rht. 



