The objectives (or lenses at the lower end of the microscope) are the most important part of the instrument, and however good it may be in all other respects, if these are defective the whole is useless. The most useful lenses for our purpose, if only two are to be selected, are a 1-in., magnifying about 50 diam., and a 1/4-in., magnifying about 200 to 400, according to the eye-piece; a 1/8-in. giving, say, twice this magnification will be needed to see the smaller bacteria distinctly, but it is possible just to see even the small putrefaction bacteria with a really fine 1/4-in. In any case, the highest power should be as perfect and of as large an angle as attainable. A good 1/4-in. should resolve _Pleurosigma angulatum_ with direct light, and should show the movement of the granules of protoplasm in the round corpuscles which are present in saliva. In using the latter test, it must be remembered that the motion only lasts a very short time on a cold slide. About 5_l._ is the very least for which a microscope can be obtained which is suitable for tanners' use; where it can be afforded, a better one is advisable. Without disparaging other makers, it may be mentioned that the writer has generally used both the eye-pieces and objectives of Dr. Hartnack of Potsdam; and that they are moderate in price, at least for the dry combinations, and perfectly satisfactory for all technical purposes. Numbers 2, 5, and 8 objectives with No. 3 eye-piece, are sufficient for all ordinary work. If only 2 objectives are to be obtained, Nos. 3 and 7 would be perhaps the best selection. It is always better to use objectives on the stand, and with the eye-pieces for which they are intended, but in case Hartnack's objectives are used on an English stand (which is easily done by means of an adaptor ring), it is important to remember that they are constructed to work with a shorter tube than that customary on English microscopes, and that they will not perform well if its length is much more than 6 in.; these objectives are not provided with a movable adjustment for thickness of cover-glasses, which for technical purposes is not required, and in inexperienced hands is apt to prove troublesome. Extra-thin covers must therefore always be used. Where this adjustment is provided, the object must be accurately focused, and then, maintaining this focus with the fine focusing-screw, the collar must be cautiously turned till the best definition is obtained. Practically it will be best to make this adjustment accurately once for all, and to take care to use covers selected of a uniform thickness.