Backgrounds 



DRY WIIOLEMOUNTS 



13 



is better to substitute either a plastic or a 

 tin cell. 



Most plastic cells seem to be stamped 

 out of vulcanite, though there is no reason 

 why the numerous other plastics available 

 today sliould not be used. It is almost im- 

 possible to punch cells from sheets of 

 plastic more than He of an inch thick 

 without special machinery, so that it is 

 better to buy them than to prepare them 

 one's self. Excellent cells may, however, 

 be prepared by anybody in possession of a 

 lathe by buying extruded tubing, readily 

 available in many types of plastic, and 

 cutting from it lengths of the appropriate 

 thickness. Opaque plastic should never be 

 used to prepare cells more than i^g of an 

 inch high, since the opaque wall interferes 

 with the illumination of the contained 

 object. Homemade cells are better pre- 

 pared from tin than plastic, since an 

 ordinary hammer and punch may be used 

 to cut sheet tin, or sheet pewter, up to a 

 thickness of nearly }i of an inch. When 

 cells have been punched from sheet, rather 

 than turned from a tube on a lathe, they 

 will be found to have a turned-down edge 

 where the punch came through. This edge 

 must be removed before they are cemented 

 by placing the cell on a sheet of fine sand- 

 paper — sandpaper blocks sold for sharpen- 

 ing draftsmen's pencils are excellent — and 

 rubbing it backward and forward until a 

 visual inspection shows that all the under- 

 surface has come in contact with the 

 abrasive. 



Cementing of cells to glass depends far 

 more for its success on the cleanhness of 

 the glass than on the cement selected. 

 Gold size has the tremendous advantage 

 that it will adhere firmly to slightly dirty 

 glass, but it has the disadvantage that 

 three or four days are required before it is 

 sufficiently firm to continue mounting. If 

 one is prepared to take the trouble to 

 clean the glass thoroughly, almost any of 

 the cements given in Chapter 28 under 

 V 11.2 may be employed. To make a firm 

 bond with liquid cements it is best to turn 

 a ring of the cement in the appropriate 

 place on the slide with a turntable, and to 

 apply a thin coat of cement to the under- 

 side of the cell. When both these coats are 

 dry another thin coat is applied either to 



the slide or the cell. The two are then 

 pressed together and dried under pressure. 

 An easy way to apply this pressure is to 

 take a 500-gram brass weight, which is 

 usually to be found somewhere about the 

 laboratory, and lay it carefully on top of 

 the cell; or one can place another slide on 

 top of the cell and clip the two slides to- 

 gether with a strong spring paper clip. 

 Whatever method, or cement, is em- 

 ploj^ed, each slide must be inspected after 

 it is dry to make sure that it is adhering 

 perfectly over all, or almost all, of the 

 base ; minute air holes will admit moisture 

 and lead to molding of the contained speci- 

 men. If the cell has been fixed with a 

 cement under pressure, a small quantity 

 of surplus cement will almost always have 

 been extruded both outside and inside at 

 the point of contact of the ring. That 

 which has been extruded outside should 

 be left in position, unless there is a great 

 deal too much of it, but the material 

 inside should be carefully scraped off with 

 the edge of a scalpel. It is most unwise to 

 endeavor to remove this cement with a 

 solvent which is hkely to loosen the cell. 



Selection of a Background 



No background other than the glass it- 

 self is necessary when the object is to be 

 prepared as a transparent wholemount; 

 but many objects prepared as a dry whole- 

 mount are better displayed against a black 

 or colored background. This background 

 may either be a varnish apphed to the 

 bottom surface of the cell, or a disk of the 

 appropriately colored paper cemented in 

 position. The best black paper is that used 

 to wrap photographic plates, but colored 

 papers of all tj^pes are available. The 

 paper is punched into disks which are at- 

 tached to the bottom of the cell with any 

 adhesive. They will not be subjected to 

 any strain, and office mucilage, or any of 

 the formulas given in Chapter 28 under 

 V 11.1 will be found satisfactory. If a 

 black background is to be painted in 

 place, the most satisfactory paint is the 

 optical dead black, listed by some scien- 

 tific suppHers or available occasionally 

 from scientific instrument makers. The 

 only one of these which may be prepared 

 at home is the formula of Martin 1872 



