230 USE OF THE MICBOSCOrE. 



To convert Paris Lines into English Measure. 



Multiply the numerator of the fraction -^-^ by the number 

 of Paris lines stated, or divide the denominator of the fraction 

 by the same nmnber, or multiply the number .088815 by the 

 number of lines and parts of the same. 



To convert Millimetres into English Measure. 



Multiply the number .039371 by the number of millimetres 

 and parts of the same, the quotient will be the equivalent 

 measure in decimal parts of the English inch. 



The line is often made use of in scientific works in this 

 country ; but as no two persons are agreed as to whether its 

 ^•alue is the one-tenth or one-twelfth of an inch, it follows 

 that, in all measures in which it is employed, the value 

 attached to it should be stated ; if the Paris line be the one 

 adopted, neither the one-tenth or one-twelfth of an English 

 inch is its correct value, although the latter number comes 

 nearest the truth. 



CHAPTER V. 



CAMERA LUCIDA. 



The camera lucida, before described at page 144, was invented 

 by Wollaston in 1807. It consists of a four-sided prism of 



glass, as represented in sec- 

 tion by fig. 149, the sides and 

 angles being similar to those 

 shown by A B C D. The rays 

 of light proceeding from an ob- 

 ject, MN, after being reflected 

 by the faces, D C, C B to the 

 eye will be referred by an ob- 

 server to m n, and an image 

 of the object will be there 



