56 SELfcCllON AND USE 



capable of showing a great deal that is interesting and instruc- 

 tive. Two or three years ago these lenses were the only ones 

 furnished with microscopes costing less than $50, and in the 

 very cheap instruments the different powers were always 

 obtained by the division of one doublet or triplet, which 

 was thus made to yield two or three different objectives. 

 Those, however, who cannot afford American objectives, and who 

 wish to do work that is of some real value, are advised never to 

 separate their objectives, or at least never to separate any but 

 the very lowest that is the No. 1, and against even this 

 we would protest were it not for the fact that cheap lenses 

 of lower power than the half -inch are seldom found in market ; 

 and therefore, no other course except the division of a No. 1 

 is left to us when we wish to use a lens of lower power. But 

 this system of dividing is often carried too far, and we find, mi- 

 croscopes in market which are furnished with No. 2 or No. 

 3 objectives which are divided when lower powers are needed. 

 This is decidedly wrong. If a power lower than No. 1 be needed, 

 it may be admissible to divide this number, because this is in 

 general the only course left to us, but a No. 2 should never 

 be divided for the purpose of obtaining an objective equivalent 

 to a No. 1. 



The value of the numbers assigned to the different French ob- 

 jectives varies according to the fancy of the maker, but those 

 of the better class usually found in market are about as follows : 



Number 1 2 3 4 5 6 



Corresponding focus in parts of an inch 1-2 1-4 1-6 1-8 1-10 1-12 



Frey, in his recent work on the microsccpa, regards the 

 English system, whereby the focus is expressed in inches, as 

 " peculiar." It certainly is "peculiarly " definite and positive, 

 instead of being indefinite and arbitrary, as is the system 

 adopted by the French and German opticians. According to 

 the English and American systems, an objective of an inch 

 focus ought to be the same, no matter by what maker it has 

 been constructed, but when designated after the plan which 

 Frey seems to prefer, it is impossible to tell what the focus of 

 the lens may be, and consequently what its power is. Thus 

 a No. 2 of Nachet has a focus of half an inch, while a No. 

 2 of Hartnack has a focus of one inch, and a No. 2 of the 



