MAGNIFYING APPARATUS. 



is not desirable, however, in any case, to push the amplifying 

 power of the instrument too far, because the illumination of the 

 image in that case becomes inconveniently faint ; and if there be 

 any causes of aberration uncorrected in the lenses, whether sphe- 

 rical or chromatic, their effects will be rendered more apparent. 



7. In the mounting of the instrument, provisions are necessary 

 for varying, within certain limits, the distance of the object, as 

 well from the illuminating as from the amplifying lenses. If the 

 object be very minute, it is necessary that it should be illuminated 

 with proportionate intensity; and, therefore, that it should be 

 moved very near to the focus of the illuminating lens, c c. If it be 

 larger, this position would, however, be unsuitable, inasmuch as 

 the light would be collected upon a small part of it, to the exclu- 

 sion of the remainder. In that case, therefore, the object must be 

 brought farther in advance of the focus, F, of the illuminating lens, 

 so as to intersect the cone at a point of greater section, and thus 

 to receive a light which, though less intense, will be diffused 

 over its entire surface. 



The amplification required will be greater in proportion as the 

 object is smaller. For very minute objects, therefore, the ampli- 

 fying lens must be brought nearer to the object, and the screen 

 must be removed farther from it, while for larger objects, the 

 arrangement would be the reverse. 



8. All that has been said on the subject of the screen in the 

 case of the magic lantern will be applicable to the solar micro- 

 scope, except that, in this case, the method of showing the object 

 througfi. a transparent screen is objectionable, because of the light 

 which is lost by it, and for. other reasons; and, besides, it is 

 useless, that method of exhibition being adapted only for phan- 

 tasmagoria, and other similar subjects of amusement. 



9. In what has been explained above, it has been assumed that 

 a beam of solar light is thrown upon the condensing lens c c, in 

 the direction of its axis. Now it is evident that it could never 

 happen that the natural direction of the sun's rays would coincide 

 with that of the axis of the tube of the microscope ; for, that axis 

 being necessarily horizontal, or nearly so, the sun to throw its rays 

 parallel to it should be in the horizon. Some expedient, there- 

 fore, is necessary, by which the direction of a sunbeam can be 

 changed at will, and thrown along the axis of the tube. 



The obvious method of accomplishing this is by means of a plate 

 of common looking-glass ; such a plate being conveniently mounted 

 in front of the condensing lens, may always have such a position 

 given to it that it will reflect the sunbeam which will fall upon it 

 in the direction of the axis of the tube. 



But since, by reason of its diurnal motion, the sun changes its 



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