ACCESSORY APPARATUS. 99 



scale; or when the Camera Lucida (or any similar arrangement) is em- 

 ployed for the purpose of Micrometry. All that is requisite to turn it to 

 this account, is an accurately divided Stage-micrometer, which, being 

 placed in the position of the object, enables the observer to see its lines 

 projected upon the surface upon which he has drawn his outline; for if 

 the divisions bo marked upon the paper, the average of several taken, 

 and the paper then divided by parallel lines at the distance thus ascer- 

 tained (the spaces being subdivided by intermediate lines, if desirable), a 

 very accurate scale is furnished, by which the dimensions of any object 

 drawn in outline under the same power may be minutely determined. 

 Thus, if the divisions of a Stage-micrometer, the real value of each of 

 which is a 100th of an inch, should be projected on the paper with such 

 a magnifying power as to be at the distance of an inch from one another, 

 it is obvious that an ordinary inch-scale applied to the measurement of 

 an outline would give its dimensions in lOOths of an inch, whilst each 

 tenth of that scale would be the equivalent of a 1,000th of an inch. 

 When a sufficient magnifying power is used, and the dimensions of the 

 image are measured by the ' diagonal 7 scale (which subdivides the inch 

 into 1,000 parts), great accuracy may be obtained. It was by the use of 

 this method, that Mr. Gulliver made his admirable series of measure- 

 ments of the diameters of the Blood-corpuscles of different animals. In 

 using Nachet's vertical Camera for Microtnetry, care must be taken so to 

 adjust the slope of the drawing-board, that the Micrometer scale shall be 

 projected on the paper without distortion. 



96. Nose-piece. It is continually desirable to be able to substitute 

 one objective for another with as little ex- 



penditure of time and trouble as possible; 

 so as to be able to examine under a higher 

 magnifying power the details of an ob- 

 ject of which a general view has been 

 obtained by means of a lower; or to use 

 the lower for the purpose of finding a 

 minute object (such as a particular Dia- 

 tom in the midst of a slide-full) which we 

 wish to submit to high amplification. 

 This is effected by the Nose-piece of Mr. 

 C. Brooke, which, being screwed into 

 the object-end of the body of the Micro- 

 scope, carries two objectives, either of 

 which may be brought into position by 

 turning the arm on a pivot. In its origi- Swift ' s improved Nose-piece, 

 nal form, the arm was straight; so that the Objective not in use was often 

 brought own upon the Stage, unless the relative lengths of the two objec- 

 tives were specially adjusted. This inconvenience is avoided, however, in 

 the construction adopted by Messrs. Powell and Lealand, and further sim- 

 plified by Mr. Swift (Fig. 71); the bend given to the arm having the effect 

 of keeping the Objective not in use completely off the stage. The work- 

 ing Microscopist will scarcely find any Accessory more practically useful 

 to him than this simple piece of apparatus. 



97. Finders. All Microscopists occasionally, and some continually, 

 feel the need of a ready means of finding, upon a glass slide, the particu- 

 lar object, or portion of an object, which they desire to bring into view; 

 and yarious contrivances have been suggested for the purpose. Where 

 different magnifying powers can be readily substituted one for another, 



