218 



MICROSCOPE. 





Mr. Siv- 

 right's 

 method of 



'" e 



Dr. Brew- 

 ster's new 

 single mi- 

 croscope. 



PLATE 



CCCLXX 

 Fig. 5. 



formed plano-convex lenses of any focal length ; and, 

 jjy Cropping t h e varnjgh on both sides, he formed dou- 

 ble convex lenses, with their convexities in any requi- 

 red ratio. The focal length of these lenses incre.-ise a 

 little after they are formed, on the upper side of the 

 glass, but diminish if they are formed on the lower 

 side of it ; and if they are preserved from dust, they will 

 last a long time. Dr. Brewster employed these fluid 

 lenses as the object glasses of compound microscopes. 



A very ingenious method of forming single micro- 

 scopes of glass has been recently proposed and executed 

 by Mr. Sivright of Meggetland. Take n piece of pla- 

 tinum leaf, about the thickness of tinfoil, and make 

 two or tni ' ee circular holes in it, from one-twentieth to 

 one-tenth of an inch in diameter, and at the distance of 

 about half an inch from each other. In the holes put 

 pieces of glass, which will stick in them without fall- 

 ing through, and which are thick enough to fill the 

 apertures. When the glass is melted at the flame of 

 a candle with the blow-pipe, it forms a lens which ad- 

 heres strongly to the metal, and the lens is therefore 

 formed and set at the same time. The pieces of glass 

 used for this purpose should have no mark of a dia- 

 mond or file upon them, as the mark always remains, 

 however strongly they are heated with the blow-pipe. 

 The lenses which were made larger than one-tenth of 

 an inch were not so good as the rest, and the best were 

 even of a smaller size than one-tenth. As the lenses 

 thus formed sometimes contain air- bubbles, the best 

 way is to make several, and select those which are 

 freest from imperfections. An eye or loop, made by 

 bending the extremity of a platinum wire, may be used 

 instead of the platinum leaf. The reason of using pla- 

 tinum is, that the glass is more easily and more per- 

 fectly melted in this than in other metals, which may 

 perhaps arise from its being a bad conductor of heat, 

 and from its preserving its brightness. As platinum 

 does not oxidate, the glass adheres better to the edges 

 of the holes, and it may be used very thin, as it does 

 not melt with the heat necessary for the complete fu- 

 sion of the glass. 



Mr. Sivright has likewise succeeded in forming 

 plano-convex lenses by means of fusion. Having laid 

 a fragment of glass upon a flat and perfectly polished 

 natural surface of topaz, which is easily obtained by 

 cleavage, he exposes the whole to an intense heat. 

 The upper surface of the glass assumes a spherical 

 (igure, in virtue of the mutual attraction of its parts, 

 and the lower surface becomes perfectly flat, and highly 

 polished, from its contact with the polished plate of 

 topaz. See Edinburgh Philosnph. Journal, vol. I. p. 81. 

 A new kind of single microscopes, which possess 

 particular properties, have been lately constructed by 

 Dr. Brewster. They consist of plano-convex or dou- 

 ble-convex lenses of a hemispherical or spherical form, 

 which may be converted into single microscopes, hav- 

 ing a much greater magnifying power than when they 

 a re used in the ordinary way. Let ABC, for example, 

 be a hemispherical plano-convex lens, which may be 

 used as a single microscope, either by presenting the 

 side A or the side D to the. object. Rut if we place 

 the object at inn, and looking in at F, examine it after 

 reflexion from the surface BC, we shall have an effect 

 the same as if we had placed the two lenses A a 1; d, 

 Aacd, with their plane sides AB, AC together, or 

 rather of a double-convex lens similar to the two unit- 

 ed. As the light is incident on BDC at an angle of 

 45, it will suffer total reflexion, and not a ray of it 

 will be lost. The spherical aberration of the lens 

 used in this way, is to its spherical aberration used in 



the common way, as 97 to 117. A hemispherical lens Si/igic Mi- 

 employed in this manner, and having the line BC in- croscopc?. 

 clineil 45 to the axis of telescope, will form the best """ ~Y"'' 

 possible diagonal eye-piece. When the segment ABC 

 is less or greater than a hemisphere, the same effects 

 will be produced. With a double-convex lens, 

 shewn in Fig. 6. a very high degree of magnifying 

 power is obtained by looking in at E, and we have the Fig. 6. 

 effect of a compound microscope, consisting of an ob- 

 ject-lens AC, a concave mirror CBD, and an eye-glass 

 AC. The perfection of the image will, in this case, 

 increase, as the incident or reflected rays approach to 

 DA. In these, as well as in all other microscopes, the 

 object should be illuminated from one point, such as a 

 single candle or luminous aperture. See the Edinburgh 

 Philosophical Journal, No. V. 



A very great improvement on the microscope has Dr. Wol- 

 been lately suggested by Dr. Wollaston, under the l;>ton's pe- 

 name of the Periscopic Microscope. riscopicmi- 



" The great desideratum," says he, " in employing ^ osc *P e - 



t-i / n* fi i i i.*.* 'II. A IE 



high magnifiers, is sufficiency of light ; and it is ac- ,< CLXXVII. 

 cordingly expedient to make the aperture of the little Fig. 7. 

 lens as large as is consistent with distinct vision. But 

 if the object to be viewed is of such magnitude as to 

 appear under an angle of several degrees on each side 

 of the centre, the requisite distinctness cannot be given 

 to the whole surface by a common lens, in consequence 

 of the confusion occasioned by oblique incidence of 

 the lateral rays, excepting by means of a very small 

 aperture, and proportionable diminution of light. In 

 order to remedy this inconvenience, I conceived that the 

 perforated metal which limits the aperture of the lens 

 might be placed with advantage in its centre, and ac- 

 cordingly I procured two plano-convex lenses, ground 

 to the same radius, and applying their plane surface on 

 opposite sides of the same aperture, in a thin piece of 

 metal (as is represented by a section, Fig. 7.) I pro- 

 duced the desired effect ; having virtually a double- 

 convex lens so contrived, that the passage of oblique 

 pencils was at right angles with its surface, as well as 

 the central pencil. With a lens so constructed, the 

 perforation that appeared to give the most perfect dis- 

 tinctness was about one-fifth part of the focal length in 

 diameter ; and when such an aperture is well centered, 

 the visible field is at least as much as 20 degrees in 

 diameter. It is true that a portion of light is lost by 

 doubling the number of surfaces; but this is more than 

 compensated by the greater aperture which, under 

 these circumstances, is compatible with distinct vi- 

 sion." Pliil. Trans. 1812, p. 375. 



It is obvious, from the last paragraph of this quota- Dr- Brew s- 

 tion, that the idea had not occurred to Dr. Wollaston of tel ' s lm- 

 filling up the central apc-rture with a fluid of the same onThT"" ' 

 refractive power as the glass of which the two lenses riscopic 1 *" 

 are composed. This improvement, which was prac- microscope 

 tised by Dr. Brewster, as shewn in Fig. 8. removes en- Fig- 8. 

 tirely the loss of light arising from doubling the sur- 

 faces. The same thing may be done in a still more 

 perfect manner, as in Fig. 9- as proposed by the same F 'g- 9- 

 author, where a groove is cut round the globule or 

 spherical lens, by the wheel of a seal-engraver. By 

 this means, the doubling of the surfaces is avoided, and 

 the most perfect centering obtained. As it is much 

 more uasy to grind two double convex lenses, than two 

 piano convex ones, as shewn in Fig. 7- we might avail 

 ourselves of this circumstance in the construction, to 

 adopt the form shewn in Fig. 10. and render the mi- 

 croscope achromatic, by introducing a concave fluid 

 lens of a different refractive and dispersive power, as 

 shewn in Fig. 10. which, independent of the correction 



' 10- 



