2 2 Journal of Comparative Neurology and Psychology. 



largest size the apparatus accommodates. Since only medullated nerve 

 fibers were to be counted and since, after osmie acid, these are indi- 

 cated by the dense black deposit of the reduced oxide of osmium in 

 their medullary sheaths, it was desirable to get photographs giving as 

 much contrast as possible. For this reason, some form of the photo- 

 graphic plates used in "process work" was usually employed. These 

 also allow considerable latitude in the exposure and always give strong 

 negatives. With strong negatives, a good quality of blue print paper 

 gives excellent results for the purpose required, and this was used 

 throughout because of the simplicity of its manipulation as well as its 

 inexpensiveness. 



In making the counts of the fibers in a section, the photograph 

 of the section was pinned out upon a soft ])ine board and the identical 

 section itself placed under the compound microscope and subjected to 

 higher magnification than that under which it had been photographed. 

 After a few minutes study, any field in the photograph could be iden- 

 tified in the microscope. Then with a sharp pointed pencil, the pho- 

 tograph was divided into fields of about one inch square or less in 

 area, the lines of division passing among and between the fibers in- 

 stead of across them. The actual enumeration was done with an auto- 

 matic tallying register, to the thumb-press of which had been fixed a 

 needle holder. Each fiber in one of the outlined fields was punctured 

 by the needle and all doubtful cases were settled by reference to the 

 original under the microscope. The counting of one field was finished 

 before beginning another. The counting apparatus, its use and the 

 trustworthiness of the results obtained with it, are discussed in detail 

 in the paper cited above. The method has since been used with little 

 modification (Dunn 'go, Hatai 'c2, Inobert '03) and the results ob- 

 tained have been uniformly considered satisfactory. The method has 

 the advantage of being largely mechanical and eliminating as far as 

 possible the psychological sources of error which are most to be feared in 

 investigations of this kind. 



It was not deemed neces.sary to photograph certain of the small 

 divisions of the dorsal branches. Such as were small enough to bear 

 a magnification of 700 diameters and still not occupy the field of the 

 microscope, were counted by a modification of the more commonly 

 used net-micrometer method. The procedure followed i.*^ fully de- 

 scribed in the author's paper of 1900. By means of the projection of 

 the squares of the micrometer and the completion of the count before 

 the result is known, it is held that all errors of auto-suggestion are 

 avoided. 



