OBJECT-GLASS HOLDER OBJECT- MARKER. 127 



would be the equivalent of a thousandth of an inch. When a 

 sufficient magnifying power is used, and the scale thus made is 

 minutely divided, great accuracy may be obtained. It has been 

 by the use of this method, that Mr. Gulliver has made his admi- 

 rable series of measurements of the diameters of the Blood-cor- 

 puscles of different animals. 



50. Object- G-lass Holder. In Microscopes of the old construc- 

 tion, whose objectives were single lenses, these were not unfre- 

 quently mounted near the periphery of a circular disk pivoted 

 to the lower end of the body, in such a manner that any desired 

 power might at once be brought into use by merely rotating the 

 disk. Since the introduction of achromatic object-glasses, this 

 method has been until recently abandoned ; every "power" being 

 separately connected with the extremity of the body, so as not to 

 admit of any substitution, save by screwing off one objective and 

 screwing on another. The old method, however, has been par- 

 tially reverted to by Mr. C. Brooke ; who has contrived a holder 

 into which two objectives may be screwed, and which, being 

 attached to the u nose" of the body, enables either of them to be 

 brought into position, by simply turning the arm on its pivot. 

 This is an extremely convenient arrangement, and might easily 

 be carried further if desired ; since, by having a tri-radiate or 

 quadri-radiate arm, three or four powers might be thus brought 

 into use successively, with as much facility as two. The principal 

 objection to the general use of such an appendage, lies in the 

 nicety of workmanship that is required to obtain that exact 

 " centering," which is needed to bring the axis of the objective 

 into precise continuity with that of the body ; and unless this be 

 attained, the performance of the instrument is greatly impaired. 

 In microscopes of the old construction, the other imperfections 

 were so great, that none but an excessive deficiency in this 

 respect would attract attention. The convenience of such an 

 instantaneous change in the power of the objective is very great; 

 since it is continually desirable to obtain a general view of an 

 object with a low power, and to examine the parts of it in detail 

 under a higher amplification, with as little expenditure of time 

 and trouble as possible. 



51. Object-Marker. All Microscopists occasionally, and some 

 continually, feel the need of a ready means of finding, upon a 

 glass slide, the particular object or portion of an object, which 

 they desire to bring into view ; and various contrivances have 

 been suggested for the purpose. Where different magnifying 

 powers can be readily substituted one for another, as by the use 

 of the Erector ( 44) or of the Object-glass holder ( 50), no special 

 means are required ; since, when the object has been found by 

 a low power, and brought into the centre of the field, it is rightly 

 placed for examination by any other objective. Even this slight 

 trouble, however, may be saved by the adoption of more special 

 methods ; among the simplest of which is marking the position of 



