ON THE USE OF THE MICROSCOPE. 583 



see, by the shadow consequent on changing the illumination, whether a body is 

 concave or elevated, or whether a small body is solid or hollow. But there 

 are numerous other cases, which prove the great advantage to be derived from a 

 proper use of various forms of illumination. Great weight has justly been always 

 placed upon the regulation of the illumination in the microscope ; and although 

 many precautionary measures formerly employed, and the frequently very com- 

 plicated apparatus, have been rendered partly superfluous in modern times through 

 the great improvements of the optical part of the microscope; for instance, in the 

 use of achromatic and aplanatic lenses ; yet there still remains one point which 

 deserves great attention, and the importance of which has been very much 

 neglected by many microscopical observers. The principle laid down by Wol- 

 laston, that all light which does not immediately subserve the purpose of illu- 

 minating the object is injurious to the distinctness of vision, is a sound principle 

 for guiding our observations at the present day. It is particularly to be recom- 

 mended that all lateral light should be excluded from the eyes by a suitable 

 screen, and with transparent objects ; that all side-light should be excluded from 

 the object by means of a hollow pasteboard tube, blackened inside, reaching from 

 the body of the microscope to the table. 



In the next place, I will make a few remarks on the method of microscopical 

 investigation. The object of all microscopical investigations is to obtain as perfect 

 a knowledge of forms and processes, which, from the dimension of the object, are 

 invisible to the naked eye, as would be possible were the objects possessed of 

 dimensions equal to those of substances which we can with perfect distinctness 

 recognise by means of the naked eye. Our eye is in itself an optical contrivance ; 

 the microscope repeats nearly the same means ; and we should therefore re- 

 member that the microscope can in no way give us new qualities any more than 

 the eye itself. The function of the eye is to transmit directly to our perception 

 variously coloured and illuminated points, which are arranged in a mathematical 

 picture upon a plane surface, and we become conscious of the corporeal quality, 

 the third dimension of space, by a subsequent process of the mind. We must 

 therefore keep in view the fact, that the mode of action of the eye we mean, of 

 course, of the healthy eye is founded, like that of the microscope, upon im- 

 mutable mathematical laws ; that errors consequently are only committed by the 

 erring judgment in all observations, whether instituted with the naked eye or the 

 microscope. The healthy sense and the optical in strument are always right. 

 " Nature does all things well : confusion is only found in the heads of men." 

 We mention this in order to allude to two very common prejudices, the influence 

 of which upon science has been injurious in many respects, because they prevent 

 people from tracing error to its proper source. 



One of these consists in the vague phrase, that microscopical researches can 

 never be depended upon, inasmuch as the microscope is frequently very deceptive. 

 Such an expression, alas ! has been employed within a very recent period by 

 men who are looked up to as authorities in the natural sciences. It is easy to 

 refute this notion. The microscope is perfectly innocent of every thing of which 

 it is accused. The evil spirits which, as long as the world has existed, have 

 always impeded the advances of the human mind, and which, even at the present 

 day, and especially in the natural sciences, and still more in microscopical re- 

 searches, have caused so much mischief, are precipitation, superficial^, and we 

 may add to these scientific dishonesty, of which frivolousness is always guilty. 

 These give us occasion, with much justice, to be upon our guard when micro- 

 scopical researches are put forth ; but this is not due to the falseness of the 

 instrument, but to the untruth of men. How many persons, for instance, have 

 given erroneous impressions by attributing the colours due to chromatic aberration 

 to bodies, by describing air-bubbles as objects, &c. ; but this is not the fault of the 

 microscope, but of the stupidity, and the want of judgment arising therefrom, of 

 people who make researches with an instrument of whose laws and mode of 

 action they are ignorant, and gave their opinions about subjects of which a little 

 reflection would have taught them that they were not in the slightest degree 

 qualified to judge. 



The other prejudice is the direct opposite of the preceding, and yet it is 



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