S. E. Dowdy. M. P. S

Now, with regard to the question of the aperture of the lens, suppose, for example's sake, we were observing the minute markings on a diatom valve with a narrow apertured lens of good workmanship. If the diatom was a suitable one, we should probably see a series of fine lines close together, these becoming more apparent if oblique illumination is used. If a wide apertured lens was now substituted, though of exactly the same magnifying power, it is very probable that these lines would be shown as what they really are - vis. a series of beads or dots, thus affording an example of the value of aperture in an objective-Supposing we now replace our narrow-angled lens and tried the effect of various eyepieces on the image yielded by it, we should find that when we got to eyepieces magnifying more than about ten diameters the image would greatly deteriorate in quality and break down, as it is termed. On the same object, however, the wide-angled lens would work with much more powerful eyepieces without showing much falling off. This is, of course, explained by the theory on which images are formed in compound microscope. The primary image formed by the objective is magnified by the eyepiece ; any imperfection is, therefore, shown in proportion to the magnifying power of the eyepiece employed. This is the reason why powerful eyepieces are of little use to the student, because his lenses, owing to their want of aperture, would break down, or else not admit sufficient light with such eyepieces. The real value of aperture, therefore, is that it imparts brilliancy of image, better definition and resolving power to a lens. It should not, however, be forgotten that increased aperture means vastly increased skill to Impart the necessary corrections for the elimination of spherical and chromatic aberration; so that, unless the workmanship be really good, the increase in aperture may be more than counter-balanced by the presence to a certain extent of these defects.

If possible, it would be a good plan for the student to beg the loan of a large-apertured 1/4" lens, and try it under these conditions against his own comparatively narrow-apertured objective of the same focal length. I say "comparatively narrow-apertured," because, as before mentioned, students' lenses are now mostly made with wider angles than they used to be, owing to the recognition of the fact that increase of aperture, with good workmanship, means increase of resolving power and improvement in definition. Unfortunately for those of limited means, increase of aperture involves a big increase in price, owing to the skill and care required to eliminate spherical and chromatic aberration under such conditions. If possible, though, it is a good plan to pay the extra money asked for, and substitute a fairly wide angled 1" lens for the lower apertured one likely to be supplied with a student's stand. The increased cost will be fully repaid, because I he wide angled objective, if by a good maker, will admit more light, define better, and, what is an important point to anyone not provided with a large selection of lenses, will stand much deeper eyepiecing. It may have a shorter working distance; but that is not a very grave defect, providing the focal length is anything over 1/2". A low-power lens can be satisfactorily used, even by the beginner, except perhaps for dissecting purposes.

I should not advise the student, however, even if he can afford it, to purchase a large-apertured high-power dry lens, because such an objective, though yielding finer results in experienced hands, is not an easy thing for the beginner to manipulate. For one thing, a slight alteration in thickness of cover or length of body-tube will impair the image shown by it; but the most serious drawback would be that its working distance would be extremely short, perhaps so much so that only the thinnest of cover-glasses could be used over objects viewed through it.

The term "dry lenses " has been used several times in explaining the manipulation of the ordinary objectives the student is likely to require. The adjective is used to distinguish such lenses from those which have their fronts immersed in a liquid, such as oil or water, and which are therefore known as "immersion objectives." It must suffice to just mention their existence, as unless the student is taking up the study of bacteriology, he will not require a lens of this kind. Without therefore, entering into an explanation of the principle on which this type of lens construction is based, which would involve the introduction of such terms as refraction and dispersion, and imply a previous knowledge of optics on the reader's part for a proper understanding, it will be, perhaps, sufficient to say that immersion lenses have a greater working distance than dry ones for the same aperture, and are also less sensitive to thickness of cover-glass. Of this type of lens, 1/12" oil immersion is the one most frequently used in biological and bacteriological work. Such a lens having an aperture of 1.25 can be obtained for $25 upwards, so it will be seen that an objective alone may cost much more than a complete students' microscope. Immersion lenses are not difficult to use, it being necessary merely to put a drop of either cedar oil or water, as the case may be, on the cover-glass surmounting the object. The objective is then lowered by the coarse adjustment until its front lens touches this liquid, when focussing is completed, without removing the eye from the body-tube, by means of the fine adjustment. After use the lens-front should be carefully wiped with absorbent cotton-wool before replacing it in its case. Enormous magnifying powers up to seven thousand diameters may be obtained by the use of such lenses and suitable eyepieces ; but such magnifications are- seldom required (except in bacteriology and similiar work) and cannot always be relied upon, like the images obtained with a lower-power lens.