The opportunities for discovery and original work, which are afforded by all alike, will be more readily appreciated and utilised by adopting such a plan than by a general and purposeless distribution of effort. To mention one or two. The student of the fascinating field of pond-life will find new organisms awaiting description by the hundred, and of the old ones, life-histories to make out; if he be attracted rather to the vegetable inhabitants of the same realm, the diatoms will furnish him with the opportunity of studying, and perhaps solving, the enigma of their spontaneous movement, and of watching their development. The smaller fungi, and indeed the larger ones too, will amply repay the closest and most laborious study of their habits of life and processes of development. Since the first edition of this work was published, the whole subject has been practically revolutionised, and more remains to be done than has yet been accomplished. In short, there is scarcely an organism, even of those best known and most studied, which is so completely exhausted that persevering investigation would reveal nothing new concerning it. There can be little doubt but that if any worker, with moderate instrumental means, but with an observant mind, were to set determinately to work at the study of the commonest weed or the most familiar insect, he, or she, would by patient labour accomplish work which would not only be of value to science, but would redound to the credit of the worker. Something like finality appears to have been reached, at least for the present, in the development of the microscope; and whilst it is beyond the scope of this work to treat of the refined and costly apparatus which is essential to useful work in certain departments of research, the result has, on the whole, been highly favourable to the worker of moderate means and ambitions, since lenses are now accessible, at the cost of a few shillings, comparatively speaking, which could not have been purchased at all when this work first appeared. It is with such appliances that we have here to deal. The worker whose finances are restricted must be contented to extemporise for himself many pieces of apparatus, and will find pleasure and occupation in doing so. And let him remember, for his encouragement, that many such home-made appliances will fulfil their purpose quite as well as the imposing paraphernalia of glittering brass and glass which decorates the table of the wealthy amateur. It is not the man who possesses the best or most costly apparatus, but the one who best understands the use of that which he possesses, that will make the most successful microscopist. A good observer will discover, with only the aid of a pocket-magnifier, secrets of Nature which have escaped the notice of a whole army of dilettante microscopists, in spite of the advantages which, as regards instruments, the latter may enjoy.