EYE-PIECES. 



lenses, having their plane sides turned towards the eye ; they are 

 set in opposite ends of a brass tube, varying in length from two 

 inches downwards, according to their focal lengths, the distance 

 between them and, consequently, the length of the tube being 

 always equal to half the sum of their focal lengths. 



The higher the power of the eye-piece, and consequently the 

 shorter the focal length of the eye-glass, the less will be the length 

 of the tube in which they are set. 



This tube is called the EYE- PIECE. 



It will be apparent from what has been explained, that the 

 magnifying power of the instrument will depend conjointly on 

 those of the object-piece and eye-piece. 



21. In the prosecution of microscopic researches, the use of 

 very various magnifying powers is indispensable ; the higher 

 powers would be as useless for the larger class of objects, as the 

 lower ones for the smaller. But even for the same object, a 

 complete analysis cannot be accomplished without the successive 

 application of low and high powers : by low powers the observer 

 is presented with a comprehensive view of the entire form and 

 outline of the object under examination, just as an aeronaut who 

 ascends to a certain altitude in the atmosphere obtains a general 

 view of the country, which would be altogether unattainable 

 upon the level of the ground. By applying successively higher 

 powers, as has been already explained, the smaller parts and 

 minuter features of the object are gradually disclosed to view, 

 just as the aeronaut, in gradually descending from his greatest 

 altitude, obtains a view of objects which were first lost in the 

 distance, but at the same time loses, by too great proximity, the 

 general outline. 



The microscope-makers, therefore, supply in all cases an 

 assortment of powers, varying from 30 or 40 upwards ; observa- 

 tions requiring powers under 40, being more conveniently made 

 with simple microscopes. For this purpose it is usual, with the 

 best instruments, to furnish six or eight object-pieces and three 

 or four eye-pieces, each eye-piece being capable of being combined 

 with each object-piece. The number of powers thus supplied will 

 be equal to the product of the number of object-pieces, multiplied 

 by the number of eye-pieces. 



The powers, however, may still be further varied, by provi- 

 sions for changing the distance between the object and eye-pieces, 

 within certain limits. For this purpose, the tube of the instru- 

 ment is sometimes divided into two, one of which moves within 

 the other, like the tube of a telescope, the motion being produced 

 by a fine rack and pinion : in this case the eye-piece is inserted in 

 one of the tubes, and the object-piece in the other. By combining 



25 



