HARD WICKE 'S S CIENCE - G O SSIP. 



i75 



avoided : nothing should be included but with the 

 object of use — the more useful the more beautiful it 

 will be ; two pieces of goodly-sized stone or irregu- 

 larly shaped brick should be placed nearly touching 

 the front glass ; these stones soon become covered 

 with organisms, and may be so adjusted as to be 

 within range of an ordinary magnifying lens. Two 

 or three plants (not more) of vallisneria, or chara, 

 are to be planted in one corner in well- washed gravel, 

 banked up with one of the pieces of stone ; the other 

 part of the floor of the tank should be left bare, to 

 facilitate the picking up, free of sand or gravel, of 

 anything that may appear. If a pond be accessible, 

 three parts of the tank are filled with water from it ; if 

 it be cloudy or even muddy the result may be more 

 favourable. After being left in repose for at least a 

 week, the character of the contents (if the pond be 

 fairly productive) will be seen ; the water will be 

 clear and probably reveal a variety of common objects, 

 such as larva; of insects, entomostraca, planaria, and 

 hydra; ; in a week or more vegetable growth will 

 appear, covering every portion of the interior. The 

 tank is now in a condition to be inoculated with 

 whatever choice objects can be obtained, the result 

 of special and favourable gatherings, and this should 

 be repeated frequently ; the front glass must be occa- 

 sionally cleaned with a sponge tied to the end of a 

 cane, but on no account be tempted to touch the 

 sides or back : the water will soon become as clear as 

 crystal, however muddy it may have been when first 

 introduced. The pieces of stone will show signs of 

 vegetable growths, with patches of such forms of in- 

 fusoria as vorticella; and stentors. The waste from 

 evaporation must be supplied by additions of pond 

 water (the richer the better), and such a tank will be, 

 in a month or two (not before), a marvel of micro- 

 scopic beauty and interest. The untouched back 

 glass will be covered with a dense mantle of dark- 

 green velvety vegetation, in the midst of which will 

 be discovered groups and patches of the fixed 

 infusoria, and it is essentially the habitat of the 

 polyzoa; hydra?, and the rarer rotifera philodina, 

 even melicerta and^stephanoceros, may be found under 

 such circumstances, to say nothing of the countless 

 tribes of free infusoria, ever ready for observation in 

 all their various phases of existence, and such con- 

 ditions will preserve and increase them indefinitely. 

 Enemies to eliminate are larva; of insects (but these 

 soon disappear naturally), the fluviatile arachiada;, 

 and the larger molluscs; the latter in browsing through 

 the vegetation on the glass are apt to destroy perhaps 

 a favourite group of stentors. Other enemies are the 

 floating filamentous alga; ; they should be removed, 

 but if the tank be well watched as regards light no 

 trouble in this respect may ensue. Nothing ought 

 to he planted in the middle, as it is of importance the 

 back glass should be clearly seen, and much may be 

 lost if anything impedes this view. It needs hardly 

 be said that the water must never be changed. 



Tanks so established will greatly improve by time, 

 even when it extends to years. It is worthy of note, 

 as a matter of practical observation, that a great 

 excess of animal life (of a certain character) much 

 conduces to the development and well-being of 

 microscopic life. In a tank measuring fourteen 

 inches deep by fourteen wide and twenty-eight inches 

 long, five full-grown Mexican axolotls (nearly as 

 large as water-rats) have existed and thriven for 

 more than four years. In every part of this tank there 

 are swarms of Crustacea, infusoria, and rotifera, and 

 the back glass facing a wall is covered with the velvet- 

 like growth, shading off into patches of pale browns 

 and purples, imbedded in which are considerable 

 masses of living animal objects. It would seem as 

 if the rejectamenta of these axolotls (they are fed 

 once a week with strips of raw beef) is conducive 

 to the development of life. In large ordinary tanks 

 with growing plants of vallisneria, and not devoted 

 to the special object of these notes, it is desirable to 

 have a piece of floating wood ; it will in a few weeks 

 or months form the nidus or habitat of many strange 

 organisms. Vegetable forms requiring running or 

 moving water, as desmids, vol vox, &c, cannot be 

 reared or even kept in captivity. Much might be 

 said of collecting to supply tanks with objects did 

 space admit, but hydra; and countless forms may be 

 procured in abundance by the very simple process of 

 bringing in a handkerchief-full of duckweed, washing 

 it thoroughly in a pan, rejecting the weed, and 

 pouring the resulting water into the tanks. 



Crouch End. E. D. 



HOW TO USE THE MICROMETER. 

 By F. Kitton, Hon. F.R.M.S. 



ALL interested in microscopic studies have been 

 more or less inconvenienced by the frequent 

 absence of a scale of measurement attached to the 

 figures ; this is a great blemish in that otherwise 

 valuable work, " The Microscope," by Dr. Carpenter. 

 This in many cases was no doubt unavoidable, no 

 scale having been given with the original figures. As 

 the measurement of microscopic objects is by no means 

 difficult, eveiy one using the microscope should make 

 it a rule to ascertain the dimensions of the objects 

 he is examining. The only additional apparatus 

 required is some form of camera lucida : personally I 

 prefer Wollaston's ; others give the preference to 

 Beale's neutral-tint camera lucida (the former 

 costs 21s., the latter 7s. 6d.), and a micrometer ruled 

 in txits a "d r ^ ls . The chief difficulty in using the 

 camera of Wollaston is bisecting the pupil of the 

 eye with the edge of the prism ; if this is not care- 

 fully attended to either the paper or the object be- 

 comes invisible : practice soon overcomes the diffi- 

 culty. In using either form of camera it is necessary 

 that the body of the microscope should be horizontal. 



