MOUNTING OPAQUE OBJECTS. 319 



passed in the direction shown by fig. 215 ; the discs may 

 be made black with lamp-black (that sold in 

 T shops in the moist state in little oblong saucers 



J will be found the best) or with lacker in which 



^^^ lamp-black has been mixed; in this latter case 

 ^J^QHV they should be warmed either before the lacker 

 ^^^ is apphed or afterwards, to dry it. The felt 

 which is used as gun-wadding, or the pellets that 

 are sold already cut out for guils, may be substi- 

 tuted for the card-board and chamois leather, or 

 even leather itself may be used with advantage, 

 ig. 215. Transverse slices of small phial corks are very 

 good, but to make them look well, they should have their cut 

 surfaces covered with black paper, which renders their manu- 

 facture rather more troublesome. 



Upon these discs the objects are to be cemented ; this may 

 be readily done either with some thick lamp-black or with the 

 lacker and lamp-black, both these cements having 

 the advantage of being a dull black, and not of a 

 shiny aspect as gum or sealing-wax dissolved in 

 spirits of wine, which, on this account, are objec- 

 tionable; the darker an object is, the more dark 

 ought the disc to be ; white discs should be 

 avoided, as they reflect the light and interfere with 

 p. correct definition. Objects may be placed upon 



both sides of the discs, or one side may be oc- 

 cupied by a number for the sake of reference, and this 

 side may be either left white or black ; if black, the number 

 may be put on in white, or a printed one with a white margin 

 may be used. Five different ways of mounting objects on these 

 discs are shown by figs. 215-16-17. In the first, where the 

 objects are thick, they may be simply cemented to the disc. 

 In the second is seen a plan which answers very well for the 

 capsules of mosses, viz., to glue a small piece of cork to the 

 lower surface of the disc, and to attach the little stems of the 

 capsules to this ; they can then be arranged in the best way 

 for viewing their mouths. In the third and fourth ways the 

 same thing is shown, but a small circle of cork is employed 



