CH. VIII '] PHOTO-MICROGRAPHY 213 



ranged, one should be certain that the light brings out the details most 

 desired. For example, if one is photographing an embryo which 

 shows the branchial pockets well, great pains should be taken to so 

 arrange the embryo with reference to the light that the proper shading 

 will be given to bring out the gill pockets most emphaticall}'. One can 

 learn to do this only by practice. It is advantageous to have an assist- 

 ant, then while the operator is looking into the camera the assistant 

 can turn the embryo in various directions until the appearance is most 

 satisfactory. 



§ 347. Focusing and Making the Exposure. — For getting a 

 general focus, and for the general arrangement the ground glass screen 

 is used, but for the final focusing it is desirable to use a clear glass 

 screen and a focusing glass. In this way one can focus as satisfactor- 

 ily as with an ordinary microscope. In daylight with white embryos 

 and a dark ground 30 to 40 seconds is usually sufficient exposure. One 

 must learn this also by trial and it facilitates the obtaining of exact data 

 to make a record of every negative made, whether the negative is good 

 or bad. A table is given in § 360 to facilitate the record taking. In a 

 short time one can learn to make the correct exposure. If the result 

 is unsatisfactor)-, try again. The rule adhered to by all first rate 

 workers is to to stick to it until the result is satisfactory. 



§ 34S. Records of Embryos. — Each specimen or litter of speci- 

 mens will have its own label giving date and method of preparation. 

 It is an advantage to write this label with water-proof carbon ink, then 

 one can'put the label in the dish with the embryos and it will form a 

 part of the picture and serve as a record. 



After the picture is satisfactorily made it is wise to number the 

 embryos on the back of the negative with a wax crayon, and later 

 when the negative is dr5' number on the front with carbon ink. The 

 embryos are placed in separate bottles each with a copy of the original 

 label and the number corresponding with that put on the negative. 

 This is easily accomplished if the embryos are arranged in definite 

 rows as advised in § 346. 



Finally when the embryo is cut into serial sections and mounted, 

 a picture of the whole embryo should accompany the series. 



§ 349. Size of the Pictures. — For all embryos it is well to 

 make one picture natural size (x 1) and then for the smallest ones a 

 magnification of at least five times natural size (X5). Here, as with 

 the magnification of the microscope, linear magnification is always 

 meant (§ 154-155). 



