4:0 THE MICROSCOPE AND ITS REVELATIONS. 



CHAPTER II. 

 CONSTRUCTION OF THE MICROSCOPE. 



41. THE optical principles whereon the operation of the Microscope- 

 depends having now been explained, we have next to consider the mechani- 

 cal provisions whereby they are brought to bear upon the different 

 purposes which the instrument is destined to serve. And first, it will be 

 desirable to state those general principles which have now received the 

 sanction of universal experience, in regard to the best arrangement of its 

 constituent parts. Every complete Microscope, whether Simple or Com- 

 pound, must possess, in addition to the lens or combination of lenses 

 which affords its magnifying power, a stage whereon the Object may 

 securely rest, a concave mirror for the illumination of transparent objects 

 from beneath, and a condensing -lens for the illumination of opaque objects. 

 from above. 



I. Now, in whatever mode these may be connected with each other, 

 it is essential that the Optical part and the Stage should be so disposed, as 

 either to be altogether free from tendency to vibration, or to vibrate together; 

 since it is obvious that any movement of one, in which the other does not 

 partake, will be augmented to the eye of the observer in proportion to the 

 magnifying power employed. In a badly- constructed instrument, even 

 though placed upon a steady table resting upon the firm floor of a well- 

 built house, when high powers are used, the object is seen to oscillate so* 

 rapidly at the slightest tremor such as that caused by a person walking 

 across the room, or by a carriage rolling-by in the street as to be fre- 

 quently almost indistinguishable: whereas in a well-constructed instru- 

 ment, scarcely any perceptible effect will be produced by even greater dis- 

 turbances. Hence, in the choice of a Microscope, it should always be 

 subjected to this test, and should be unhesitatingly rejected if the result 

 be unfavorable. If the instrument should be found free from fault when 

 thus tested with high powers, its steadiness with low powers may be 

 assumed; but, on the other hand, though a Microscope may give an image 

 free from perceptible tremor when the lower powers only are employed, it 

 may be quite unfit for use with the higher. The Author has found no 

 test for steadiness so crucial as the vibration of a paddle-steamer going at 

 full speed against a head-sea; and the result of his comparison between 

 the two principal 'models' generally used in this country will be stated 

 hereafter (49). 



ii. The next requisite is a capability of accurate adjustment to every 

 variety of focal distance, without movement of the object. It is a principle 

 universally recognized in the construction of good Microscopes, that the 

 stage whereon the object is placed should be & fixture; the movement by 

 which the focus is to be adjusted being given to the optical portion. 

 This movement should be such as to allow free range from a minute fraction. 



