A very great improvement on the microscope has been lately suggested by Dr. Wollaston, under the name of the Periscopic iNlicrcscope.
" The great desideratum," says he, " in employing high magnifiers, is sufficiency of light ; and it is ac cordingly expedient to make the aperture of the little 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, Plate CCCLX XV I I. Fig. 7,) I produced 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, 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 vision." Phil. Trans. 1812, p. 375.
It is obvious from the last paragraph of this quota tion, tbat the idea had not occurred to Dr. \Vollaston of filling up the central aperture with a fluid of the same refractive power as the glass of which the two lenses are composed. This improvement, which was prac tised by Dr. Brewster, (as shewn in Plate CCCLXXVII.
Fig. 8.) removes entirely the loss of light arising from doubling the surfaces. The same thing may be done in a still more perfect manner, as in Fig. 9. as proposed by tile same author, where a groove is cut round the glo bule 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 easy to grind two double convex lenses, than two plano-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 drferent refractive and dispersive power, as shewn in Fig. 10. which, independent of the correction of colour, is a simpler construction than that shewn in Fig. 7.
Following out the principles which we have here ex plained, Dr. Brewster has proposed a miscroscope, con strutted as in Plate CCCLXXVII. Fig. II, where AB, CD, are two double convex lenses, so constructed that the concave lenticular space ABDC, would, when filled with a fluid of different dispersive and refractive power, correct the aberration of refrangibility. A con vex speculum of steel or polished silver, perforated ith an aperture c f, has its anterior surface P e highly polished, and either attached by a transparent cement or not to the lens All. The aperture e f being filled with the proper fluid, and CD cemented to the posterior part of the steel, the lens is ready for use. The curvature of the reflector P e Q, is such as to re flect parallel rays incident on AB, upon the object to be examined, being converted into a concave speculum by the lens placed before it.
By this means, we obtain a lens possessing very unusual properties, and capable or affording the most perfect view of microscopic objects.* Having thus described the various kinds of single microscopes which have been used, we shall now give an account of the improvements which have been made in adapting them to the examination of minute objects.