THE MICROSCOPE. 



magnifying power is so considerably diminished, that the defects 

 of the image produced by the unbalanced aberrations are very 

 inconsiderable, and the observer is generally content to tolerate 

 them on account of the great economy gained by the separation of 

 the lenses, which supplies, without additional expense, three 

 independent object-pieces. 



Some of the foreign makers, less scrupulous in the exact ad- 

 justment of their optical combinations, make all the three lenses 

 composing each triple object-piece exactly similar, unscrewing 

 one from another, so as to enable the observer to use one, two, or 

 three at pleasure. It is evident that, with such combinations, the 

 aberrations can never be so exactly balanced as they are in the 

 object- pieces above described ; but in the instruments to which 

 they are applied, powers exceeding 700 or 800 are almost never 

 attempted, and the aberrations, though imperfectly compensated, 

 are sufficiently so to prevent much injurious confusion in the 

 images. 



In the superior class of instruments, where magnifying power 

 is pushed to so extreme a limit as 1500 or 2000, the most extreme 

 precision in the balance of the aberrations must necessarily be 

 realised, since the slightest imperfection so prodigiously magnified 

 would become injuriously apparent. 



The extreme degree of perfection, which has been attained in 

 the best class of microscopes, may be imagined, when it is stated, 

 that an object which is distinctly visible under a power of 1500 

 or 2000, when it is exposed to the object-glass uncovered, will be 

 sensibly affected by aberration if a piece of glass, no more than 

 the 100th of an inch in thickness, be laid upon it. Infinitesimally 

 small as is the aberration produced by such a glass film, it is 

 sufficient, when magnified by such a power, to be perceptible, and 

 to impair in a very sensible manner the distinctness of the image. 



As it has been found necessary, for the preservation of micro- 

 scopic objects, to cover them with such thin films of glass, through 

 which, consequently, they are viewed, adjustments are provided 

 in microscopes with which the highest class of powers are supplied, 

 by which even the small aberration due to these thin plates of 

 glass thus covering the objects can be corrected. This is effected 

 by mounting the lenses, which compose the triple object-piece, 

 in such a manner that their mutual distances, one from another, 

 can be varied within certain small limits, by motions imparted to 

 them by fine screws. This change of mutual distance produces a 

 small effect upon the aberrations, rendering their total results 

 negative to an extent equal to the small amount of positive 

 aberration produced by the thin glass which covers the object. 



20. The eye-glass and the field-glass are both plano-convex 

 24 



