The next step in the improvement of the simple microscope beare more analogy to the eye-piece. This improvement was made by Mr.
Holland, and it consists (ass shown in fig. 6) in anbatituting two lenses for the first in the doublet, and retaining the stop -- between them and the third. The first bending, being thus effected by two lenses instead of one, is accompanied by smaller aberrations, which are there fore more completely balanced or corrected at the second bending, in the opposite direction, by the third lens. This combination, though called a triplet, is essentially a doublet, in which the anterior lens is divided into two. For it must be recollected that the first pair of lenses merely accomplishes what might have been done, though with less precision, by one ; but the two lenses of the doublet arc opposed to each other; the second diminishing the magnifying power of the first. The first pair of lenses in the triplet concur in producing a certain amount of magnifying power, which is diminished in quantity and corrected as to aberration at the third lens by the change in relation to the position of the axis which takes place in the pencil between what, is virtually the first and second lens. In this combination the errors are still further reduced by the close approximation to the object which causes the refractions to take plaee near the axis. Thus the transmission of a still larger angular pencil, namely 65°, is rendered compatible veitledistinctness, and a more intense image is presented to the eye.
Every increase in the number of lenses is attended with one draw back, from the circumstance that a certain portion of light is lost by reflection and absorption melt time that the ray enters a new medium. This lose bears no sensible proportion to the gain arising from the increased aperture, which, being as the square of the diameter, multiplies rapidly; or if we estimate by the angle of the admitted pencil, which is more easily ascertained, the intensity will be as the square of twice the tangent of half the angle.
To explain this, let 13 n (fie. 7) represent the
diameter of the lens, or of that part of it which is really employed ; C A the perpendicular drawn from its centre, and A A D, the extreme rays of the incident pencil of light D A D. Then the diameter being 2 Cu, the area to which the in tensity of vision is proportional will be (2 c and c D is evidently the tangent of the angle c a 14, which is half the angle of the admitted pencil A B. Or, if a be used to denote the angular aperture, the expression for the intensity is (2 tan. b a)', which increases so rapidly with the increase of a, as to make the loss of light by re flection and absorption of little consequence.
The combination of three lenses approaches, ae has been stated, very close to the object; so close indeed as to prevent the use of more than three ;. and this constitutes a limit to the improvement of the simple microscope, (for it is called a simple microscope, although ' consisting of three lenses, and although a .compound microscope may be made of only three or even two lenses;) but the different arrange ment which gives rise to tho term compound will be better under stood when that instrument is explained.
Before we proceed to describe the simple microscope and its appendages, it will be well to explain such other points in reference to the form and materials of lenses as are most likely to be interesting.
A very useful form of lens was proposed by Dr. Wollaston, and called by him the periscopic lens. It consisted of two hemispherical lenses, cemented together by their plane faces, having a stop between them to limit the 'aperture. A similar proposal was made by Mr.
Coddington, who, however, executed the project in a better manlier, by cutting a groove in a whole sphere, and filling the groove with opaque matter.
Ills lens, which is the well-known Coddingtou lens, is shown in fig. 8. It gives a large field of view, which is equally good in all directions, as it is evident that the pencils A A' and u n' pass through under „precisely the same circumstance& ata sphe.