42 PRACTICAL TREATISE ON 



screwed ring, C, or more simply by sliding the one on the 

 other, and clamping them together. When adjusted, an 

 aperture is made in the tube. A, within which is seen a mark 

 engraved on the cylinder, and on the edge of which are two 

 marks, a longer and a shorter, engraved upon the tube ; when 

 the mark on the cylinder coincides with the longer mark on 

 the tube, the adjustment is perfect for an uncovered object, 

 and when the coincidence is with the short mark, the proper 

 distance is obtained to balance the aberrations produced by 

 glass one-hundredth of an inch thick, and such glass can 

 readily be obtained. When Mr. Ross first effected this im- 

 provement, he made the adjustment by sHding the outer tube, 

 A, upon the cylinder, B ; but Mr. Powell, we are told, was 

 the first to apply the screw coUar, C, by which the correction 

 can be performed with greater nicety, and Mr. Smith after- 

 wards, as a refinement, added a graduation to it. Mr. Ross, 

 however, has found that for the adjustment to be perfectly 

 correct, it must be tested experimentally. 



The method of using this improved achromatic object-glass 

 wiU be again alluded to in the chapter devoted to the com- 

 pound microscope. From the peculiar construction of Mr. 

 Ross's higher powers, he is enabled to transmit extraordinarily 

 large angular pencils of light: on several occasions he has 

 obtained the enormous aperture of 135. 



Mr. PoweU, in early life, was engaged in the manufacture 

 of philosophical instruments, but not of microscopes; and it 

 was only in the year 1834 that he devoted his attention to the 

 last mentioned instruments. In the same year, we find a con- 

 tribution of his to the fiftieth volume of the Transactions of 

 the Society of Arts, entitled, " On a fine adjustment for the 

 Stage of a Microscope." This ingenious contrivance was 

 applicable to any instrument, but Mr. Powell used it with the 

 adjustable stage made by Mr. Turrell, and described by him 

 in the forty- ninth volume of the same transactions. The 

 slow movement was obtained by making the stage stand on 

 three feet, under which three inclined planes were moved 

 simultaneously by one screw, a single turn of which raised or 

 lowered the stage only the three-hundredth part of an inch, 



