8 PRACTICAL MICROSCOPY, 



but inverted image of the candle will be thrown upon the 

 screen at a d, and the further A B is placed away from the 

 principal focus for parallel rays, the smaller will be a b, and 

 the nearer will it be formed to the opposite principal focus. 

 On the other hand, if a b represents the lighted candle 

 placed in front of the principal focus F but very near to it, 

 the real image which is formed, A B, is much amplified, in 

 proportion as a b is nearer to the principal focus, and the 

 distance at which A B is formed is likewise regulated by 

 the distance of the object from this point. The nearer a b 

 is to F, the further will A B be formed away from the lens, 

 and vice versd. 



The student will now be able to see the principle upon 

 which the microscope is constructed, and, in closing this 

 chapter, we do so by giving a table of the magnifying 

 power of single convex lenses of varying focal lengths, for 

 parallel rays at a distance of 12 J inches from the micro- 

 meter to the screen, which has been constructed from a 

 paper upon the subject by Dr. Woodward in 'Silliman's 

 Journal' for June 1872. 



Focal Distance Amplification 



for in 



Parallel Rays. Diameters. 



i" .;. ...... 10-4 



...... 23-0 



F ........ 48-0 



F ...... .. 98-0 



T y ........ 198-0 



