HARDWICKE'S SCIENCE-GOSSIP. 



81 



is capable of effecting very satisfactorily all that 

 can be required of it. The wheel is formed out of 

 a, brass pulley about three eighths of an inch thick, 

 having a groove around its circumference to receive 

 a large gut band by which it is rotated. The screw- 

 head and washer in the centre are sunk below the 

 level, in order to avoid the lubricating grease getting 

 on the slides, while the whole sm-face has been made 

 as true as possible. The two pins b and b sup- 

 port the slide exactly half an inch from the centre, 

 while the pin A stops the slide at exactly an inch 

 and a half from the right-hand end : the slide 

 being pressed firmly down by the spring above, is 

 thus kept in its position, and is prevented from shift- 

 ing during use. It is here apparent that all devia- 

 tion frona tlie standard inch by three inches will 

 present itself at the sides d and e, where it will 

 not interfere with the centring, nor can it be suf- 

 ficient to be in any way detrimental to appearance. 

 The preceding may be taken to represent the 

 normal arrangement of the table, but when required 



Fig. 61. Wheel of Turntable, i nat. size. 



for accommodating misplaced cells, the two pins a 

 and B require to be taken out, when the end of the 

 slide E falls down upon the " cam " f, which is a 

 screw with a flat eccentric head of the form shown 



s 



the thin line indicating a piece of mainspring for 

 greater steadiness. Then by turning the screw f 

 aud pushing the slide to the right or left, any cell or 

 mounting may, witli a little practice, be readily 

 brought into the centre; when it will be firmly held 

 by the spring above it. This centring may be 

 greatly facilitated by having a few circles made on 

 the brass surface of the wheel beneath the slide. 



The foregoing adjustments may be very readily 

 obtained in the following manner. First, describe a 

 circle o/ie inch in diameter in the centre, by rotating 

 the table on its own axis. Next, draw a line quite 

 across, just toucJdtig the outer edge of the circle, and 

 let the holes for the two pins be drilled, not on the 

 line, but just so mucli below as will bring the edge of 

 .ihe holes up to the line as accurately as possible. 

 Lastly, as the radius of the circle is just half an inch. 



another inch beyond this circle will give the exact 

 distance for the line and pin A, equal care being 

 taken to have the latter beyond and up to the line, 

 but not on it. Now, if every turntable were to be 

 readjusted to this standard, namely, one and a half 

 inch from the right-hand end and half an inch 

 above the lower edge, slides and turntables would 

 be rendered as interchangeable as modern stands 

 and object-glasses have become through the adop. 

 tion of the universal screw. 



But it is not for "cells" alone that this centring 

 is here pleaded ; it is equally desirable that every 

 object, even in balsam or dammar, or in any other 

 medium, should be so placed that its cover may be 

 cemented down for greater security, and which may 

 readily be done in a very simple manner. Having 

 placed the slide in its position, draw upon it a circle, 

 with ink, as large as the slide will admit of, as at 

 c, making a dot opposite a for future guidance. 

 (I use " Draper's dichroic ink," because it flows 

 freely, is a brilliant black, and does not corrode or 

 clog steel pens ; but its greatest merit for this pur- 

 pose is, that when thoroughly dried the glass may 

 be afterwards cleaned with spirit of wine with- 

 out disturbing it, yet it may be removed by 

 the slightest touch of a piece of wood wetted 

 with liquor potassa : it may be had of almost 

 any stationer at sixpence the half-pint bottle.) 

 Having made the ink circle on the tipper side of the 

 glass, then with a pair of compasses, make a similar 

 circle on a piece of cardboard ; when dry, turn the 

 glass over with the ink circle next the card, placing 

 the two circles concentrically, and mark a dot or 

 small ring around the central spot indicated by the 

 point of the compass, or a second glass slip marked 

 with a ring and central dot may be used instead 

 of the card. This latter central spot will 

 now be exactly under the spot corresponding with 

 the centre of the turntable, and over which the ob- 

 ject should be placed, the large circle being left till 

 the last, as a guide for the final centring of the 

 cover before completion. 



W. Kencely Bridgman. 



AMONG THE ANDAMANS. 



TN the Bay of Bengal, between the 10th and 14th 

 •-*- parallels of north latitude and the 92nd and 

 94th degrees of east longitude, lie the beautiful 

 tropic islands of the Andamans, known to us since 

 the Indian mutiny chiefly as a penal settlement, 

 but latterly painfully associated in our minds with 

 the mournful tragedy enacted there on the 8th 

 Eebruary, 1872. 



The Andamans proper consist of four large 

 islands aud a multitude of smaller ones, mostly 

 covered with luxuriant forest, and almost every- 

 where locked in a fringe of coral, which in many 



