Answers to Queries. 153 
achromatic, as an unachromatic one is useless for modern require- 
ments. Compound microscopes may be roughly divided into 
three classes :—Firstly, the complete and expensive instruments of 
Powell and Lealand, Ross, Beck, Tolles, etc. etc., which are 
chiefly suitable for the examination of ready-prepared slides, reso- 
lutions of diatoms, and all work which wants the highest qualities 
of lenses to be brought out with the greatest ease and efficiency. 
Secondly, cheap, flashy instruments, with long tubes, many wheels, 
and various appliances, got up in imitation of the first class, but of 
inferior workmanship. ‘These are made for sale, not for use, and 
should be carefully avoided. Thirdly, working instruments of 
simpler construction, but of good workmanship. Of these there 
are many kinds. But it is always advisable to get one of these at 
first, as, even if the purchaser contemplates getting a first-class 
instrument, it will always be of use for ordinary work afterwards. 
In this latter case, one of the cheap, small patterns, with 5- or 
6-inch tube, would be most handy. 
For full details on microscopes, I must refer R. C. to Carpen- 
ter on “ The Microscope,” Davis’s “ Practical Microscopy,” and 
other of the larger works, and also to the dealers’ catalogues. 
But a few remarks may be of service here. Microscopes are 
either binocular or monocular. The usual form of binocular 
microscope in England is Wenham’s, and this can be instantly 
converted into a monocular form if required. The advantages of 
a binocular for popular work—such as insects, etc.—is very great ; 
besides, it gives a stereoscopic effect. For the higher magnifica- 
tions or for making out small details, the monocular form is 
preferable. Monocular microscopes are made in two patterns— 
one with the long English tube, 8 to 10 inches, and the other with 
the short, foreign tube, 5 to 6 inches. This latter form is prefer- 
able when much work has to be done with an upright position (as 
when liquids are used on the stage), and is, therefore, much used 
by medical students. If this form is bought, the eye-pieces are 
better if corrected for the short tube. The chief points to be 
observed in buying an instrument is to see that the stand is steady 
in all positions, and it ought, for general work, to have a joint to 
incline the instrument, as the continual use of the upright position 
is fatiguing to most persons. The fine motion ought to work 
smoothly in every way and gradually, without shifting the object 
laterally ; the coarse motion may bea sliding-tube arrangement, 
but a rack and pinion is certainly better. The stage ought to be 
roomy, and I should recommend a plain stage without mechanical 
movement, except, perhaps, a rotatory one, though this is not so 
practically important as might be thought; the body ought to 
have a draw-tube, and two eye-pieces ought to be got magnifying 
the initiatory power of the objective (got by dividing 10 inches by 
