The following passage is taken from Marvels of Pond-life by Henry James Slack on the use of the microscope.
The microscope is rapidly becoming the companion of every intelligent family that can afford its purchase, and, thanks to the skill of our opticians, instruments
5 which can be made to answer the majority of purposes may be purchased for three or four guineas, while even those whose price is counted in shillings are by no means to be despised. The
10 most eminent English makers, Wales, and Tolles, in America, and Hartnack, in Paris, occupy the first rank, while the average productions of respectable houses exhibit so high a degree of
15 excellence as to make comparisons invidious. We shall not, therefore, indulge in the praises of particular firms, but simply recommend anyreader entering upon microscopic study
20 to procure an achromatic instrument, if it can be afforded, and having at least two powers, one with a focus of an inch or two thirds of an inch, and the other of half or a quarter. Cheap microscopes
25 have usually only one eye-piece, those of a better class have two, and the best are furnished with three, or even more.
The magnifying power of a compound microscope depends upon the focal
30 length of the object-glass (or glass nearest the object), upon the length of the tube, and the power of the eye- piece. With regard to object-glasses, those of shortest focal length have the
35 highest powers, and the longest eye- pieces have the lowest powers. The body of a microscope, or principal tube of which it is composed, is, in the best instruments, about nine inches long, and
a draw tube, capable of being extended six inches more, is frequently useful. From simple optical principles, the longer the tube the higher the power obtained with the same object-glass; but
45 only object-glasses of very perfect construction will bear the enlargement of their own imperfections, which results from the use of long tubes; and consequently for cheap instruments the
50 opticians often limit the length of the tube, to suit the capacity of the object- glasses they can afford to give for the money. Such microscopes may be good enough for the generality of purposes,
55 but they do not, with glasses of given focal length, afford the same magnifying power as is done by
instruments of better construction. The best and most expensive glasses will not
60 only bear long tubes, but also eye- pieces of high power, without any practical diminution of the accuracy of their operation, and this is a great convenience in natural history
65 investigations. To obtain it, however, requires such perfection of workmanship as to be incompatible
with cheapness. An experienced operator will not be satisfied without
70 having an object-glass at least as high as a quarter, that will bear a deep eye- piece, but beginners are seldom successful with a higher power than one of half-inch focus, or thereabouts, and
75 before trying this, they should familiarise themselves with the use of one with an inch focus.
It is a popular error to suppose that enormous magnification is always an
80 advantage, and that a microscope is valuable because it makes a flea look as advanced, but one of the noticeable often smiled at the exclamations of casual visitors, who have been pleased
85 with his microscopic efforts to entertain them. “Dear me, what a wonderful instrument; it must be immensely powerful;” and so forth. These ejaculations have often followed the use
86 of a low power, and their authors have been astonished at receiving the explanation that the best microscope is that which will show the most with the least magnification, and that accuracy
95 of definition, not mere increase of bulk, is the great thing needful. Scientific men always compute the
apparent enlargement of the object by one dimension only. Thus, supposing an
100 object one hundredth of an inch square were magnified so as to appear one inch square, it would, in scientific parlance, be magnified to “one hundred diameters,” or one hundred linear; and
105 the figures 100 would be appended to any drawing which might be made from it. It is, however, obvious that the length is magnified as well as the breadth; and hence the magnification of the whole
110 surface, in the instance specified, would be one hundred times one hundred, or ten thousand: and this is the way in which magnification is popularly stated.