666 LABORATORY EXERCISES 



9. All drawings and diagrams should be accurately and intelligibly labeled. 

 Generally it is also desirable that the parts of the drawing, especially the parts of a 

 diagram, be designated in a way that is convenient for reference. 



10. Drawings should be made either entirely in ink, or entirely in pencil, and the 

 lettering also, which should be uniform, not one style, then another. 



11. Large headings should be more especially emphasized by larger letters, and 

 the lettering of the larger and smaller headings should be of the same style. 



12. All drawings presented to the teacher for examination should be placed 

 between the two sides of a folder of stiff manila paper. 



13. The grade of pencil should be determined by the kind of finish or surface of 

 the drawing paper, but in general for science work, the harder grades of lead, say 

 from 4H to 6H, are preferable. 



14. The name of the student, the number and the subject, as well as the year, 

 should in all cases be placed on the outside of the manila cover. 



Method and Materials of Photomicrography (Fig. 236). — The photographic 

 plates which best meet the requirements in photomicrographic work with the 

 Edinger apparatus are Lumier Sigma 9 by 12 cm. plates, or the ordinary 4 by 5 

 plates. Another good plate is known to the trade as Seed Special 27. 



Whatever plate is used, it is placed in the plate holder of the photomicrographic 

 camera in a dark room, the dull side of the plate being outermost. The holder is 

 then placed in its proper position in the photographic camera. Before the insertion 

 of the holder, however, the object to be photographed must be focussed on the 

 ground-glass plate of the camera until a sharp image is obtained, then the focussing 

 screw should be moved a trifle, say one of the divisions of the screw, so that the 

 object is focussed up a slight amount. The light being regulated properly, the 

 exposure is made by withdrawing the shutter of the plate holder. The length of 

 time to expose the plate can be determined only by several trials until the operator 

 learns the length of time by the experience thus gained. 



The most satisfactory developer is made as follows: 



Rodinol, i part. 

 Water, 12 parts. 

 Potassium bromide, 10 drops of 10 per cent, solution. 



The advantage of this developer is that the process is sufficiently slow, so that the 

 operator may be able to study the photograph, as it makes itself evident. 



After washing in water, the negative is placed in a rather strong hyposulphite 

 solution as a fixing bath. The advantage of rodinol over metol is that the develop- 

 ment is more even and sure. Where the photomicrographs have been made ob- 

 scure, or where it is desirable to convert them into outline drawings for diagrammatic 

 purposes the following method can be used. 



Draivings on Photographic Prints. — All pen-and-ink drawings of photographic 

 prints must be made with water-proof India ink after which the photographic part 

 is bleached out by exposure for a few minutes in water containing cyanide of potash 

 (i : 500, more or less). The drawings should be exposed in this bath as long as 

 necessary. If any part of the print refuses to bleach, it should be moistened with 



