A Method of Adjusting Tube Length. . 123 



that the point of correct adjustment is indefinite, and the personal 

 equation very great. 



2. The method employing similarity of images above and below 

 focus (Aiuslie's method) applies a definite criterion ; the personal 

 equation is therefore relatively small. It can be applied in almost 

 all cases met with in practice, and is of special value for work 

 on Diatoms. It requires no apparatus other than a good fine- 

 adjustment. The information given concerns the adjustment of 

 tube length only ; little or no inference can be drawn as to the 

 correction of the zones of the lens system of different N. A. 



3. Method employing movement of image with alteration of 

 zone (Author's method) employs a very definite test, and there- 

 fore has the advantage of a low personal equation. It requires 

 special apparatus ; this is however of a simple nature. It can be 

 used for investigating the correction of the different zones of an 

 objective or condenser, and it is this property which gives the method 

 its principle advantage. For this purpose the Abbe test plate 

 forms the best object. Difficulties may be encountered when the 

 method is used on Diatoms. With ordinary slides of an histological 

 or bacterioloojical nature this does not occur. 



This method has been elaborated in order to give data of the 

 convection of different zones of microscopic objectives. Details of 

 the technique will be described in a future paper. 



Eeference. 

 1. AiNSLiE. — Photomicro. Soc, 1R16, p. 23. 



