178 



THE AMERICAN NATURALIST [Vol. LV 



-f 4.2 and — 2.5. The modifiers of a cross bred may be indi- 

 cated in some appropriate manner as [+4.2, —2.5]. 



S, Linkage is best represented by the fractional form used by 

 M-orkers on Drosophila. The factors are written in the order of 

 linkage, omitting type factors. 



Committee on Genetic Form and Nomenclature 



STANDARDIZED :\IICROPHOTOGRAPHY 

 Second Contribution: The Object Factor 



In my first contribution to the subject of standardized micro- 

 pliotography, published in the Anatomical Record, I have pointed 

 out the variables and the methods which I have pursued in treat- 

 ing them. One, or perhaps more correctly a group of variables 

 were, however, left out of consideration quite purposely because 

 of the difficulty in finding for them a standard of permanent 

 value. I have in mind the microscopical section itself or what 

 may be properly called the object factor. The following four 

 elements enter into its composition: (1) the thickness of the 

 section, (2) the light absorption coefficient of the tissue, (3) the 

 relative luminosity of the different stains and (4) the depth or 

 intensity of staining. The second and third of these component 

 elements may be disregarded since experience shows us that ex- 

 posure is very little influenced by them. There remain, how- 

 ever, the first and fourth, and to determine the influence of these 

 on exposure the following experiments were undertaken. 



First of all, to avoid all possible error, slides were chosen of 

 uniform thickness measured with a Cieeri Smiths Patent Mi- 

 crometer so as to be sure that the distance of the section from 

 the substage condenser should in every case be the same. The 

 cover-glasses were also of uniform thickness. The stomach of a 

 frog preserved in Zenker's fixing fluid was sectioned into series 

 of 5, 10 and 20 microniillimeter thick sections on a Minot micro- 

 tome and care was taken to have in each case a ribbon of 100 

 even sections, thus more or less assuring their uniform thickness. 

 The sections were stretched on distilled water heated over a 

 flame and no cement of any kind was used. On one slide three 

 sections of each thickness were placed. On other slides sections 

 only of one kind were placed and their thickness marked in 

 every case by a carborundum pencil. 



