S. E. Dowdy. M. P. S.

Microscope lenses are subject to three principal defects. They may show spherical aberration, chromatic aberration, or want of flatness of field. For the causes and means of remedying these faults the student should read up an elementary book on optics, as it is only misleading to attempt to explain the why and wherefore of these characteristics of a defective lens in a few words. They are due, if present in an objective, to faulty workmanship, and should, therefore, act as danger signals, warning a prospective purchaser from acquiring the lens in question. I am, of course, referring to what are generally known as "achro-matic " lenses, such as are in general use, and not t6 the recently introduced series of lenses known as "apochromatics," the outstanding corrections of which are made by special compensating eyepieces. The presence of marked spherical aberration in an objective would be shown by the impossibility of getting a perfectly sharp image of an object, the margins of which would apparently be hazy, even when the lens should apparently be in focus. Chromatic aberration makes its presence known by imparting a halo or colored fringe to the outlines of an object seen through any lens possessing this undesirable property. Flatness of field is present if objects lying in the same plane are in sharp focus, in whatever portion of the field of view they may be. For instance, if we were looking at a piece of paper with very minute printed matter upon it, using, say a 1 inch lens and a low-power eyepiece, and the print in the centre of the field was a sharp focus, that at the edges should also be so without altering the focus; otherwise the lens does not possess a flat field, which is an important desideratum in a low-power lens. Owing to vastly improved methods of manufacture, both English and Continental makers are now supplying students with well-corrected lenses at a low price. Glaring defects are, therefore, not often to be met with, and the student is hardly likely to meet with good examples of either chromatic or spherical aberration when testing his lenses, provided they are by a maker who has a reputation to maintain. Still, lenses constructed from even the same formula will vary to a certain extent, so that it is as well that the beginner should have a general idea of how a lens may be tested. If really interested in this branch of the subject, he should obtain two or three uncorrected lenses, of varying convexity, which, with the help of the aforementioned book on optics, can be made the groundwork for several very instructive experiments bearing on spherical and chromatic aberration. After a few such experiments with stops and diaphragms of various kinds, he will better understand the value of the aperture and other important parts of his objectives.

We now come to what, for the beginner, will prove a slightly more difficult matter, viz., the testing of his high-power lens. For simplicity's sake I assumed that in testing the low-power lens, the student dispensed with the use of a substage condenser. It is impossible, however, to fairly test a high-power lens with such an inadequate illumination as that obtained by using the mirror alone. A few words, therefore, as to the method of obtaining suitable illumination with the condenser will be necessary. Most substage condensers supplied with students' instruments are what are known as Abbe illuminators, or Abbe chromatic condensers, to distinguish them from the Abbe achromatic condensers, which have carefully corrected lenses.

As the student will have no difficulty in understanding, condensers are subject to the same defects as objectives. They may, and usually do, possess both spherical and chromatic aberration, but whereas these properties in a lens would be quite sufficient to immediately condemn it, much useful work may be done with an Abbe chromatic condenser of the useful student's type, in spite of their presence. For the finest results in high-power critical work, microscopists use condensers achromatised, possessing what are termed large aplanatic apertures, but it requires a trained observer to mark their superiority over the less pretentious substitute supplied with student's instruments. When the beginner is in a position to appreciate this undisputed superiority, it will be time for him to think of exchanging his uncorrected Abbe for one of a higher type.

To return, however, to our main project, the testing of our high-power lens, which will most probably be either a 1/4" or 1/6". In the first place, to give the objective a fair trial, good illumination must be first secured, then a suitable test object placed on the stage, proper focussing effected, and allowance made for thickness of cover-glass surmounting the specimen. The tube length employed must also be the one for which the lens was corrected.

The necessary information as to thickness of cover-glass and tube length will, as' a rule, be found in the maker's catalogue, or else furnished on application ; and, if possible, the test should be made complying with these conditions. The better-class high-power dry lenses are furnished with a small adjustment, called a correction-collar, to allow for varying thicknesses of covers ; but a similar result can be obtained by the student by slightly altering the length of the draw-tube.

To secure suitable illumination, the low-power lens should be first on the instrument, and the low power eyepiece used with it. The small paraffin lamp is placed on the left side of the observer, and light from it is flashed up by the mirror. A fine ground-glass slide is now placed on the stage, and focussed with the objective. The lamp flame is now turned edge on to the mirror and the condenser is racked up until an image of the flame appears sharply defined on the ground glass. If this image is not exactly in the centre of the field, it must be made so, either by using the centering screws with which most condensers are fitted, or else by slightly altering the position of the mirror or lamp.