CONSTRUCTION AND MANAGEMENT. 



163 



when objects, not covered with a glass, were examined, but did 

 not give so good a one when the object was covered, and that 

 the clearness of the image increased and diminished according to 

 the thickness of the cover-glass. To remedy this defect, which 

 resulted from spherical aberration, Amici constructed his objectives 

 in such a manner that they could all be used with cover-glasses of 

 a definite thickness. 



"In 1837, the celebrated English optician Ross, though ignor- 

 ant of the discovery of Amici, made the same observation, and, to 

 remedy the defect, he invented correction objectives. In this kind 

 of objective, the two upper lenses (Fig. 26, d.) occupy an invariable 

 position with regard to one another. They are fixed in a movable 

 tube, and can be made to recede from, or approach, the lower part, 

 which is fixed, and carries the single or double frontal e. By turn- 

 ing the ring a, the upper lenses rise or fall, and the coiled, spiral 

 spring c regulates the small inequalities of the screw, and, above 

 all, prevents the 'back lash,' which is always produced when the 

 sense of motion is changed." 



Fig. 27 shows how a microscope, which can be inclined, is 



Fig. 27. 

 arranged for adapting thefcamera lucida. When this apparatus is 



