130 



HARDWICKE'S SC IE NC E- G O S SI P. 



many old friends will be met witl), such as tlie 

 curious Volvox, the industrious Builder x\uiinal- 

 cule, the lovely green Desmids, some of which 

 might furnish a design for a "Eeuian Star of 

 Merit," and hosts of others that are figured in 

 home publications. 



IFaiiganui, New Zealand. C. Htjlke. 



TURNTABLES AND CEMENTS. 



IN the diagram of turntable, fig. 61, p. 81, there 

 is an inaccuracy which, as there represented, 

 would render it perfectly useless. The " cam " E 

 should form a portion of a spiral with a gradually 

 increasing diameter, so as to act, in fact, like a 

 circular wedge, the piece of mainspring, which has 

 been softened at one end and turned into a loop, 

 fitting loosely on to the screw A, serving to keep 

 the slide from being moved eudwise by the fric- 

 tion in turning, as it would do were the slide and 

 the cam in direct coutact. The accompanying 

 figure is from a tracing of the original, and there- 

 fore gives the exact size and form. If the cell 

 be a little more on one side of the centre of the 

 glass slip than the other, the narrower side of the 

 glass should be placed uppermost, as this requires 

 less space than the other to be moved through in 

 adjusting. 



Erom observations recently made, there is reason 

 to believe it is not an uncommon plan to mark the 

 centre of the glass slip and then to turn it over and 

 mount the object on the under side opposite to it. 

 Such a proceeding will, of course, invert the rela- 

 tive position of the central spot with the edges of 

 the glass, and throw it out of ceutre when placed 

 object upwards on the turntable ; and from an 

 examiuation of many slides in my own collection, 

 there is full evidence of this having been the course 

 adopted. In centring, the upper surface must 

 contain the object as well as the first stage in the 

 marking. If, for instance, intending to use a half- 

 inch cover, make a circle five-eighths or three- 

 quarters of an inch in diameter, and when dry 

 place the object in the centre of this circle, and the 

 latter will be a correct guide for the cover, which 

 may very readily be adjusted with its edge equi- 

 distant within the ring, the latter disappearing on 

 clearing off the balsam. Before using, the glass 

 slip is best cleaned with spirits of wine and a piece 

 of fine cambric, after which it only requires par- 

 ticles of dust to be brushed oif immediately before 

 placing the object upon it. 



Cements may be divided, practically, into two 

 classes, — one for the making of, or fixing down, cells, 

 the other for attaching covers and fur the subse- 

 quent filling up the edges, either for security or 

 for ornamentation. Eor the present we will con- 

 fine ourselves to such cements as are to be used 



with the turntable. Eor shallow cells, either for 

 fluid or for dry objects, there is scarcely anything to 

 equal the original " black japan" ; objects mounted 

 in which nearly twenty years ago are as perfect 

 as at first. The circle should be made as full 

 and plump with the japan as possible, with a full 

 brush, and then it should be placed in a slow oven (or 

 in a Dutch oven before the fire), until baked too hard 

 to be indented with the thumb-nail. It will now 

 be found shelving off to each side of the ring, forming 

 a wedge-like edge to the inside of the cell, which 

 is very objectionable, as objects are apt to get be- 

 tween it and the cover ; therefore It must be 

 turned off to an abrupt ring, which can be done as 

 easily on the turntable as in a turning-lathe. The 

 three holes in the turntable under the left-hand end 

 of the glass slide, represented in fig. 88, admit of 

 a small conical pin being dropped in, so as to set 

 the slide fast in the only remaining direction it 

 could move. A piece of wood projects over the 

 surface of the wheel and up to within an inch of 

 the centre, to act as a rest for the finger, and then 

 with a small sharp chisel the hardened japan may 

 be scraped off' the glass and turned into shape in 

 a few moments. A quantity of these cells of all 

 diameters should be kept ready for use, as they 

 require some time in hardening. 



I 



I 



o 



Fig. 88. Portion of a Turntable. 



The material next in usefulness is termed 

 asphalte, or asphaltum, a kind of mineral pitch 

 brought from Trinidad. It is soluble in various 

 fluids, but some of these solutions have the objec- 

 tionable quality of "running in," and whenever 

 any oil, varnish, or japan gold-size, &c., is mixed 

 with it to reduce its brittleness, this tendency is 

 greatly increased. In its pure form it has none of 

 this tendency whatever, and the solvent which 

 enables it to retain the most of its original 

 character is " benzole " ; but there is this disad- 

 vantage, that if made thin enough to work freely 

 it spreads on the glass, and if thick enough to pre- 

 vent this, the benzole evaporates so fast that it 

 becomes unworkable and rubs up into lumps. The 

 desideratum has been to find some means of 

 making it workable and yet devoid of tbe running- 

 in propensity, and a suggestion made by Mr. 

 Ackland bids fair to accomplish this to its fullest 

 extent. The addition of a somewhat gelatinous 



