1910] Johnson: Quantitative Study of Salpa Chain. 



157 



the series and the serial number of the zooid. With this much 

 done it was possible to measure all of a series .'it one sitting. 

 Several little ways were found in which errors may have crept 

 into the first measurements. For instance, with so small a scale, 

 a change of focus, though very slight, often makes considerable 

 difference in the reading. One ordinarily changes the focus con- 

 stantly when looking through the microscope but it was found 

 to be possible to do without this. By keeping the same dish, 

 keeping the zooid in the same part of the dish and seeing that the 

 amount of water remained constant, the image was clear through 

 the entire series and the measurements are probably more free 

 from error. In order to avoid unconscious forcing of results, the 

 zooids were measured out of their regular order as much as pos- 

 sible. Two sets of measurements were made of each of the lasl 

 three blocks and the mean of the two was taken for the final 

 result. 



In the blocks measured, the number of individuals varies from 

 53 to 80. The measurements taken were length and dorso-ventral 

 diameter (spoken of as width). The real width being very small 



Figure 9. 



