1893.] 



MICROSCOPICAL JOURIS'AL. 



201 



and once only ; second, to enable any point in the section to be 

 recorded as precisely and definitely as a star in a constellation. 

 In the accompanying figure, A represents the regulation slide, 

 3 inches by 1 inches, lying in a recess or hollow of the carriage, 

 B, into which it fits closely and is held firmly by the clamps 

 a a. The clamps being pushed aside it can easily be removed 

 b}' its protruding corners. B, the carriage, lies firmly upon C, 

 the^ table and contains at b b, two short pins which rest equally 



1 



=a> ^ 



jE 



n 



B& 



miam 



^11 i/fifii 



B 



C'lmiiii 



B 



firmly in the two spiral grooves. These two pins will direct the 

 motion applied. The section describing exactly similar revo- 

 lutions, as the pins b b. will bring exactly the width between the 

 lines of the spiral groove under the focus as a definite part of 

 the same, decreasing in diameter as the power of magnification 

 increases. The diameter of the field and the space between the 

 lines of the spiral groove can correspond exactly only with the 



