ABSTRACT
Dr. Hannover’s book is short, which is a great recommendation, and it contains many judicious and useful observations, and is evidently the production of an experienced microscopist ; but we really can scarcely find in it much that should entitle it to appear in an English dress, as it is quite evident that the author is totally unacquainted with the present state of the microscope in this country, however familiar he may be with those of foreign construction. He does not mention, and appears to be unacquainted with any English microscope maker except Mr. Pritchard. Were he aware of what they have done, and still are daily doing, towards the perfection of the microscope as an instrument of scientific research, Dr. Hannover would surely have referred to the labours of Ross, Powell, and of Smith and Beck ; men who, with the aid of Mr. Lister, have done more in the improvement of the achromatic microscope than all the continental opticians together, and whose instruments, both for optical perfection and mechanical contrivances are, as we believe, unrivalled by those of any country in the world. But we are indeed surprised that his learned English editor should have been content thus to ignore the merits of his countrymen, and to omit all notice of some of the most important improvements in the microscope and its appliances, which have now been long in daily use. Many other proofs of Dr. Hannover’s want of information on the points to which we have referred might be adduced, but we would merely, in justification of what we have said, remark, that he appears to be quite ignorant that object-glasses may be made to adjust to varying thicknesses of the glass employed to cover an object (pp. 25, 59), although the mode in which this adjustment might be made was described by Mr. Ross as long ago as 1837, and has been universally adopted by English opticians since that time. He makes no mention either of any of the more recent, ingenious, and most valuable modes of illumination, to which, for some time past, so much and such successful attention has been paid. We refer more particularly to such contrivances as Gillett’s condenser, and the parabolic reflectors of Wenham and Shadbolt.
The book, however, as we have said, contains many judicious observations and directions useful to the young microscopist ; and the preliminary observations present, in a small compass and in plain language, as good an exposition of the principal optical considerations which are concerned in the structure and use of the microscope, as can, we think, be anywhere found within the same limits.
The first chapter is devoted to an account of the simple microscope ; and chapter ii. is on the construction of the compound microscope and its accessories ; but neither in these nor in the subsequent chapters, containing directions for the use of the instrument, do we observe anything from its novelty worthy of particular notice—at all events by those who have it in their power to refer to Mr. Quekett’s much fuller and more practical work.
The author’s observations on Micrometry are good, and will be found worth consultation, as is also what he says about the modes of estimating the magnifying power of an instrument, and for calculating the amount of spherical aberration of any combination (p. 75). With reference to this he takes occasion to remark, that the same mode of observation enabled him to judge whether his eyes had remained unchanged or not during four or five years, notwithstanding the almost daily use of the microscope. He found that during that period the sight of his right eye had become only 41-10000ths =l-244th shorter; and observes, “this was satisfactory to myself, and it may also set those persons at rest who are fearful lest the use of the microscope should injure the eye “—which would be true were the alteration of the focal range of the eye the only consequence that can arise from the prolonged use of the microscope, especially by artificial light, against which we are fully in accord with Dr. Hannover, in warning all who have regard to the preservation of unimpaired visual powers.
We subjoin the following table of comparative micro-metrical measures, given by Dr. Hannover, as it may be useful for reference in our pages :—
