" The adjustment of the microscope is then perfected, if necessary, by slightly varying the dis tance between the object-glasses ; and after that is done, the length of the tube which carries the eye-pieces may be altered greatly without disturbing the correction, opposite errors which balance each other being produced by the change.
" If the two glasses which in the diagram are drawn at some distance apart are brought nearer together (if the place of A for instance is carried to the dotted figure), the rays transmitted by n in the direction of the longer aplanatic pencil of A will plainly be derived from some point z more distant than F", and lying between the aplanatic foci of , therefore (according to what has been stated) this glass, and con sequently the combination, will then be spherically over-corrected. if on the other hand the distance between A and B is increased, the opposite effects are of course produced.
" In combining several glasses together it is often convenient to transmit en under-oorreeted pencil from the front glum, and to counter act Its error by over.correction in the middle one.
•• Slight errors hi colour may In the same manner be destroyed by opposite ones; and on the principles described we not only acquire fine correction for the central ray, but, by the opposite effects at the two feel on the transverse pencil, all coma can be destroyed, and the whole field rendered beautifully flat and distinct." Mr. Lister*. paper enters into further particulars, which are not essential to the comprehension of the subject. It is sufficient to say that his investigations and results proved to be of the highest value to the practical optician, and the progress of improvement was In con sequence extremely rapid. The new principles were applied and exhibited by Mr. Hugh Powell and Mr. Andrew Roes with a degree of gnomes which had nevar been anticipated ; so perfect indeed were the corrections given to the achromatic object-glass—so completely were the errors of sphericity and dispersion balanced or destroyed—that the circumstance of covering the object with a plate of the thinnest glass or talc disturbed the corrections, if they had been adapted to an mr covered object, and rendered an objecteglass which was perfect under one condition sensibly defective under the other.
This defect, if that should be called a defect which arose out of improvement, was first discovered by Mr. Rosa, who Immediately 'suggested the Inealt19 of correcting it, and presented to the Society of Arts, in 1637, a paper on the subject, which was published in the 51st volume of their Transactions, and which, as it is, like Mr.
essential to a full understanding of the ultimate refinements of the instrument, we shall extract nearly iu full.
" In the course of a practical investigation (says Mr. Rose) with the view of constructing • combination of lenses for the objecteglass of a compound microscope, which shank' be free from the effects of aber ration, both for central and oblique pencils of great angle, I combined the condition of the greatest possible distance between the object and object-glass ; for in object-glasses of short focal length their closeness to the object has been no obstacle in many cases to the use of high magnifying powers, and is a constant source of inconvenience.
" In the improved combination, the diameter is only sufficient to admit the proper pencil; the convex lenses are wrought to an edge, and the concave have only sufficient thickness to support their figure; coneequently, the combination the thinnest possible, and it follows that there will be the greatest distance between the object and the object-glass. The focal length is h of an inch, having an angular aperture of 60% with a distance of of an inch, and a magnifying power of 970 times linear with perfect definition on the most difficult Podura scales. I have made object-glasses of an inch focal length ; but as the angular aperture cannot be advantageously increased, if the greatest distance between the object and object-glass is preserved, their use will be very limited.
"The quality of the definition produced by an achromatic compound microscope will depend upon the accuracy with which the aberrations, both chromatic and spherical, are balanced, together with the general perfection of the workmanship. Now, in Wollaston's doublets, and Holland's triplets, there are no means of producing a balance of the aberrations, as they are composed of convex lenses only ; therefore the beat that can be done is to make the aberrations a minimum : the remaining positive aberration in these forms produces its peculiar effect upon objects (particularly the detail of the thin transparent may lead to misapprehension of their true structure; but the achromatic object-glam, where the aberrations are correctly balanced, the most minute parte of an object are accurately dlapinyed, so that • satisfactory judgment of their character may be formed.