]t becomes therefore a most important problem to reconcile a large aperture with distinctness, or, as it is called, drfinition ; and this has been done in a considerable degree by effecting the required amount of refraction through two or more lenses instead of one, thus reducing the angles of incidence and refraction, and producing other effects is a small arrow to represent the object under examination, and the cones drawn from its extremities are portions of the rays of light diverging from those points and falling upon the lens. These rays, if sutlered to fall at once upon the pupil, would be too divergent to permit their being brought to a focus upon the retina by the optical arrangements of the eye. But being first passed through the lens, they are bent into nearly parallel lines, or into lines diverging from some points within the limits of distinct vision, as from o and D. Thus altered, the eye receives them precisely as if they emanated from a larger arrow placed at o D, which we may suppose to be ten inches from tIN eye, and then the difference between the real and the imaginary arrow is called the magnifying power of the lens in question.
From what has been said it will be evident that two persons whose eyes differed as to the distance at which they obtained distinct vision, would give different results as to the magnifying power of a lens. To one who can see distinctly with the naked eye at a distance of five inches, the magnifying power would seem (and would indeed be) only half what we have mourned. Such instances are however rare ; the focal length of the eye usually ranges from six to twelve or fourteen inches, so that the distance we first assumed of ten inches is very near the true average, and is a convenient number, inasmuch as a cipher added to the denominator of the fraction which expresses the focal length of a lens gives its magnifying power. Thus a lens whose focal length is one-sixteenth of an inch is said to magnify 160 times.
which will be shortly noticed. This was first accomplished in a satia. factory manner by—.
Dr.llasiosie &maid, incepted by the celebrated philosopher whoa. name it bears; It consists of two plano-convex lenses (fig. 4) having their focal lengths in the proportion of 1 to 3, or nearly so, and ee placed at a 'betimes which can be ascertained best by o e=sc actual experiment Their plane skies are pineal towards the object, and the lune of shortest focal length next , the object.
It appears that Dr. Wollaston was led to this Invention by considering that the achromatic lluyghenean see-piece, which will be hereafter described, would, if reversed, possess similer good properties as A simple microscope. But It will be evident when the eye-piece is
understood, that the circumstances which render it achromatic are very imperfectly applicable to the simple microscope, and that the doublet, without a nice adjustment of the stop, would be valueless. Dr. Wollaston makes no allusion to a stop, nor is it certain that he contemplated its introduction, although his illness, which terminated fatally soon after the pressentation of his paper, may account for the omission.
The melee of the corrections which take place in the doublet is explained in the annexed diagram (fig. 5), where ton' I3 the object, ea portion of the pupil, and D D the stop, or limiting aperture.
Wow, It will be observed that each of the pencils of light from the extremities t. te of the object is rendered excentrical by the stop, and of consequence each passes through the two lenses on opposite sides of their common axis o r; thus each becomes affected by opposite errors, which to some extent balance end correct each other. To take the pencil I„ for instance, which enters the eye at a 13 a n, it is bent to the right at the first lens, and to the left at the second ; and as each bending alters the direction of the blue rays more than the red, and moreover es the blue rays fall nearer the margin of the second lens, where the refraction, being more powerful than near the centre, corn. pensates in some degree for the greater fossil length of the second lens, the blue and red rays will emerge very nearly parallel, and of con• sequence colourless to the eye. At the same thee the spherical abet ratien has been diminished by the circumstance that the side of the pencil which passes one lens nearest the axis passes the other nearest the margin.
This explanation applies only to the pencils near the extremities of the object. The central pencil, it is obvious, would pass both lenses symmetrically; the same portions of light occupying nearly the same relative places on both lenses. The blue light would enter the second len" nearer to its axle than the red, and being thus less refracted than the red by the second lens, a small amount of compensation would take plaza, (lade different in principle and Inferior in degree to that which is produced in the excentrical pencils. In the intermediate spaces the corrections are still more Imperfect and uncertain ; and this explains the canoe of the aberration' which must of necessity exist even in the boat-mule doublet. It is, however, infinitely superior to a single lens, and will transmit a pencil of an angle of from 35 to 50' without any very sensible error". It exhibits therefore many of the usual teat objects In a very beautiful manner.