352 THE MICROSCOPE. 



zontal plane. To see other parts clearly, the distance of the inner 

 apparatus of the microscopic tube from the object must be altered. The 

 observer has to construct the form of the observed object in his mind 

 from a succession of different images thus obtained. This is soon 

 learned : other circumstances however there are, which render micros- 

 copic investigation so difficult and particular, that it requires long 

 continued use for the practical mastery of the instrument. 



The higher the degree in which an object is magnified, the weaker 

 is the illumination. Even with a hundred fold linear enlargement, a 

 body in ordinary daylight appears as if in deep twilight, and the 

 minute particles of its surface are no longer to be distinguished. Nor 

 is the matter bettered by illumination with the direct rays of the sun, 

 collected perhaps through a lens. The light can then only be thrown 

 upon the object lens in a very oblique direction, since this lens, where 

 a greatly magnified image is to be produced, must be brought very 

 near to the object. Small prominences spread deep shadows over the 

 surface. Shining spots reflect the light with embarrassing effect. 

 For practical research, where great enlargement of the object is re- 

 quired, we must have resort to an expedient for evading these difficul- 

 ties : the object to be observed must be rendered transparent or trans- 

 lucent, and be lighted from beneath, so that the rays may pass through 

 it into the microscope and thus into the eye. This will be most conve- 

 niently effected by placing the object on a glass plate over the tubu- 

 lar frame of the microscope, under the opening of which a movable 

 mirror is placed. With this we collect the light, and the mirror is so 

 directed that it throws a fascicle of reflected rays through object and 

 instrument upon the eye. 



There are no organized and only a limited number of unorganized 

 bodies which are absolutely opaque. Thinly separated layers allow 

 the light to pass sufficiently for the microscopic examination of a body. 

 It is an essential preliminary with microscopists, if the structure to be 

 examined be not in itself simple and conspicuous enough to make this 

 unnecessary, to prepare the object for examination by means of light 

 passing through it. Small organs are to be separated and stripped of 

 the enveloping textures. Of larger and more complex structures, the 

 thinnest possible sections must be taken. Often there are parts to be 

 dissected so small that the unaided eye cannot perceive them. Their 

 division under the microscope has to be effected with the finest pointed 

 and edged instruments. Scarcely anywhere is Franklin's saying so 

 applicable as to the manipulations thus rendered necessary : " A natu- 

 ral philosopher must, at a pinch, be able to bore with the saw and to 

 saw with the auger.'*' x\s the compound microscope shows the images 

 inverted, the preparations which are to be gone through with, by means 

 of needles and knives, are highly incommodious. To remedy this, we 

 commonly use either a single microscope, or else a compound one whose 

 eye-glass represents a smaller compound one. The image, twice re- 

 versed, now appears upright. 



It is not the difficulties here indicated which have procured for mi- 

 croscopic observers the reproach, bat too well deserved, in earlier times, 

 of unreliability. Had observers always prevailed upon themselves 

 rigorously to separate what was really seen from what was only con- 



