340 



MICROSCOPE. 



tion Further, the optical disadvantages of such 

 a plan would be considerable, for the nearer to 

 the principal focus of the lens the object is 

 brought, the more obliquely will the rays fall 

 upon its surface, and the greater, therefore, will 

 be the errors of aberration. This method of 

 augmenting the power of a microscope has 

 been adopted in spite of these disadvantages,* 

 but it is not found to answer. Nevertheless, it 

 is capable of being made of great utility, as 

 we shall presently show, to a limited extent. 

 A much more generally convenient method of 

 varying the power of the microscope is to 

 employ, as object-glasses, lenses of different 

 foci ; and thus, as the same distance between 

 the image and the lens is constantly maintained., 

 whilst that of the object varies, the number of 

 times that the latter is amplified is changed in 

 a like proportion. In whatever mode addi- 

 tional amplification be obtained, two things- 

 must always result from the change; the por- 

 tion of the surface of the object, of which an 

 image can be formed, must be diminished, and 

 the quantity of light spread over that image 

 must be proportionably lessened. In the use 

 of high magnifying powers, the compound 

 microscope has the great advantage over the 

 simple, that the object need not be brought to 

 nearly the same proximity with the lens, and 

 that much more of it can be seen with comfort 

 at a time. The long focus and large aperture 

 with which the eye-piece is usually made pre- 

 vent even the prolonged use of the instrument 

 from acting prejudicially on the visual powers, 

 except in cases of peculiar tendency to nervous 

 disorders of the eye. And as the power of the 

 eye-piece as well as that of the object-glass 

 can be raised, there are scarcely any limits to 

 the magnifying power that may be obtained. 

 Practically, however, there are limits, arising 

 from the fact that, as the amplification is greater, 

 the aberrations will be increased in even an 

 augmented proportion ; so that these com- 

 pletely antagonise the benefit otherwise deri- 

 vable from the employment of high powers. 

 The aberrations can only be diminished by 

 contracting the aperture of the object-glass ; 

 and this renders the image so dark that no real 

 advantage is gained. Moreover, the imperfec- 

 tions necessary to the best compound micro- 

 scope, in which ordinary lenses are employed, 

 are further augmented by the slightest error in 

 the centering of the lenses, so that their axes 

 do not coincide. 



In addition to the two lenses of which the 

 compound microscope has been stated essen- 

 tially to consist, another is usually introduced 

 between the object-glass and the image formed 

 by it. The ordinary purpose of this lens is to 

 change the course of the rays in such a manner 

 that the image may be formed of dimensions 

 not too great for the whole of it to come within 

 the range of the eye-piece, and consequently 

 to allow more of the object to be seen at once. 



* We have seen a microscope, constructed by 

 Chevalier, in which the tube was capable of being 

 drawn out to the length of between three and four 

 feet! 



Fig. 163. 



E 



Section of Compound Microscope, with field-glass 



introduced, 



A A, the image which would be formed by the 

 object-glass alone ; B B, the image formed by 

 the interposition of the field-glass F F ; the whole 

 of this image is within the range of vision of the 

 eye-glass E E, and the field of view is therefore 

 increased. 



Hence it is called the field-glass (fig. 163). 

 It may be so adjusted, however, in regard 

 to the eye-glass, as to correct its errors in 

 almost a perfect degree; and it is now, therefore, 

 usually considered as belonging to the ocu- 

 lar end of the instrument, the rye-glass and 

 the field-lens being together termed the eye- 

 piece. Various forms of this eye-piece have 



