226 USE OF THE MICROSCOPE. 



add a cipher to the denominator of the fraction which 

 expresses the focal length of the lens, and the result will be 

 the magnifying power. Thus, if the lens be half an inch in 

 focal length, the magnifying power will be twenty diameters ; 

 if one quarter, than forty diameters ; and if one inch, then ten 

 diameters, and so on. When, however, the focal length of a 

 lens is very small, it becomes a difficult task to measure 

 accurately its distance of focus. In such cases, says Mr. 

 Eoss,* " the best plan to obtain the focal length for parallel 

 or nearly parallel rays is to view the image of some distant 

 object, formed by the lens in question, through another lens 

 of one inch solar focal length, keeping both eyes open, and 

 comparing the image presented through the two lenses with 

 that of the naked eye. The proportion between the two 

 images so seen will be the focal length required. Thus, if the 

 image seen by the naked eye is ten times aa large as that 

 shown by the lenses, the focal length of the lens in question 

 is one-tenth of an inch. The panes of glass in a window, or 

 courses of bricks in a wall, are convenient objects for this 

 purpose. In which ever way the focal length of the lens is 

 ascertained, the rules given for deducing its magnifying power 

 are not rigorously correct, though they are sufficiently so for 

 all practical purposes, particularly as the whole rests on an 

 assumption, in regard to the focal length of the eye, and as 

 it does not in any way affect the actual measurement of the 

 object." 



In the preceding account, we have estimated the magni- 

 fying power in diameters, or according to the measure termed 

 linear; but as every object is magnified in breadth as well 

 as in length, it follows that, if it were drawn as broad as it 

 is long, a very different idea of its measure would result : — 

 thus, suppose a, in fig. 148, to represent an object of its 

 natural size, and that it be required to represent the same 

 when magnified four times in length and four times in breadth, 

 the square h c d e will give such view; and if this be compared 

 with the original object a, it wiU be seen that there are sixteen 



Article "Microscope," Penny Ci/clopcedia. 



