TWO NEW INSTRUMENTS FOR BIOLOGISTS. 

 By N. a. Cobb. 



(Plate vn.) 



The Differentiator.* 



The differentiator is an instrument which was invented to assist 

 in avoiding to the greatest possible extent those annoying and 

 often destructive contractions which occur in delicate organisms 

 while they are being killed and preserved. These contractions 

 fall into two groups : (1) those which occur before death, due to 

 the action on the organism of the fixation fluids, (2) those occurring 

 during the process of bringing the organism into the fluid in which 

 it is to be finally preserved. Among the latter is an often 

 unavoidable contraction due to transferring to alcohol of succes- 

 sively increasing strength. This contraction takes place with 

 something like equality in the different parts of the organism and 

 is very slight, — much slighter than is commonly supposed, because 

 other contractions or distortions are confounded with it, the whole 

 being denominated shrinkage. Only when the distortions and 

 breakages due to diflusion currents are annihilated does one beain 

 to see how insignificant the unavoidable and true shrinkage 

 really is. 



The differentiator is made from glass tubing having an internal 

 diameter varying^ from five millimeters upwards. 



According to the nature of the fluids in use, the instrument 

 takes one of the two forms illusti-ated in Figs. 1 and 2. As will 

 be seen, a or «', the reservoir, and b, the ol)ject-cylinder or object-box 

 as well as c, the filter, are three pieces of glass tubing joined together 



* This instrument has been already described in the report (1SS9) of 

 Prof. Sladen, Secretary of the British Association Committee appointed to 

 manage the Association's table at the Naples Zoological Station. A descrip- 

 tion also appeared in the American Naturalist, August, 1889. I have since 

 made improvements which render a new description with figures not in- 

 excusable. 



