ON THE USE OF THE MICROSCOPE. 585 



the savage, over whom, from his infancy, the instinctive conceptions of nature 

 have prevailed. 



But all these various relations are foreign to the microscope. We invariably, 

 when using it, see with but one eye, generally in a state of rest, and always in a 

 certain given position in relation to the object. We also see the object always in 

 an isolated condition, and cannot therefore form a notion of it by a comparison 

 with impressions from other objects. Further, our eyes possess a certain power 

 of accommodation to different distances, not confined within very narrow limits; 

 we can see objects equally distinct, although they may be at unequal distances 

 from our eye, and we receive our visual impressions in such rapid succession, 

 that it is an easy matter for us to combine all these impressions. This also, for 

 the most part, is wanting in using the microscope, especially with a high 

 magnifying power (and also the more accurately the instrument has been finished), 

 as we only see a mathematical surface. This is particularly the case in the com- 

 pound microscope, where we do not look at a real object, but merely an image, 

 and there is therefore, for the moment, no other object of sight existing excepting 

 this mathematical surface, and the power of accommodation of our eye is of no 

 use in seeing what may be placed above or below this surface (which is in a 

 manner the profile of the object under examination), but we are compelled to 

 annihilate the one object of sight, and to substitute another in its place. It will 

 easily be conceived how infinitely this must increase the difficulty of combining 

 the separate impressions into one corporeal whole. 



Taking the whole of these remarks into consideration, the results will be 

 firstly, that there is a difference between vision with the naked eye and with the 

 microscope ; and, secondly, the fundamental principles from which rules for the 

 conduct of microscopical examination must be sought. In the first place, the 

 instinctive knowledge of the material world is made manifest to us in the percep- 

 tion of form previous to the mathematical conception, for which latter the eye, as 

 the organ of sight, only furnishes us with some few elements, whilst we receive 

 the rest from other senses ; the conception derived by the other senses is entirely 

 lost in microscopical objects, and the elements furnished to us by the eye are, 

 moreover, divided during microscopical observation ; the separate parts are 

 isolated, and presented to us under circumstances which infinitely increase the 

 difficulty of their combination. Secondly, in order to avoid these disadvan- 

 tages, and to secure the results of microscopical researches against the errors of 

 judgment arising from the exercise of the faculty of mathematical conception, 

 we must endeavour to increase the number of elements in such a manner as to 

 gain thereby, as much as possible, a perfect and safe foundation for the perception 

 of form. 



This task involves the necessity of examining thoroughly every aspect of 

 the same object, and of removing from it everything which does not belong 

 to it. This last part of the task is partially accomplished by improvements in 

 the instrument, in as far as they obviate errors of form and colour (which are 

 based upon the spherical and chromatic aberration). Respecting these two 

 points, which concern the optician more than the observer, we have already 

 before mentioned every thing necessary, and the only concern of the observer 

 ought to be to procure himself the best possible instrument. Besides these, 

 however, there are many other optical phenomena, of which the observer should 

 be conscious, which, although belonging to the image, yet do not belong to the 

 object observed, and with which every one ought to be acquainted, in order to 

 be able to eradicate their share in forming our conception of the nature of the 

 object. To such belong many of the phenomena of colour which are not pro- 

 duced by chromatic aberration. The bending of the rays of light not unfre- 

 quently occurs in using the microscope. On observing, for instance, verj' small 

 holes, perhaps pores of the cellular walls, if the object does not happen to lie 

 exactly at the right distance from the object-glass, the internal surface will appear 

 coloured, and, according to the size of the pore, or the distance from the focus, 

 yellowish, reddish, or greenish. During the observation of very small globules, or 

 other solid substances, a delicate coloured border will be seen under similar cir- 

 cumstances. But both phenomena disappear if the object is brought exactly 

 within the right focal distance. We should therefore always endeavour to get rid 



