102 APPENDAGES TO THE MICROSCOPE. 



over the prism itself. When once a good position has been ob- 

 tained, the Eye should be held there as steadily as possible, until 

 the tracing shall have been completed. It is essential to keep in 

 view that the proportion between the size of the tracing and that 

 of the object is affected by the height of the eye above the paper ; 

 and hence that if the Microscope be placed upon a support of dif- 

 ferent thickness, or the Eye-piece be elevated or depressed by a 

 slight inclination given to the body, the scale will be altered. — 

 This it is, of course, peculiarly important to bear in mind, when a 

 series of tracings is being made of any set of objects which it is 

 intended to delineate on a uniform scale ; or when the Camera 

 Lncida (or any similar arrangement) is employed 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 pro- 

 jected upon the surface upon which he has drawn his outline ; for 

 if the divisions be marked upon the paper, the average of several 

 be taken, and the paper be then divided by parallel lines at the 

 distance thus ascertained (the spaces being subdivided by inter- 

 mediate lines, if desirable), a very accurate scale is furnished, by 

 which the dimensions of any objects 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 1 -200th 

 of an inch, should be projected with such a magnifying power as 

 to be at the distance of an inch from one another on the paper, it 

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

 of an outline, would give its dimensions in two-hundredths of an 

 inch, whilst each fifth of that scale would be the equivalent of 

 one-thousandth of an inch. When a sufficient magnifying power 

 is used, and the dimensions of the image are measured by the 

 ' diagonal ' scale (which subdivides the inch into 1000 parts), 

 great accuracy may be obtained. It has been by the use of this 

 method, that Mr. Gulliver has made his admirable series of mea- 

 surements of the diameters of the Blood-corpuscles of different 

 animals. 



73. Nose-piece. — It is continually desirable to be able to substi- 

 tute one Objective fbr another with as little expenditure of time 

 and trouble as possible ; so as to be able to examine under a higher 

 magnifying power the details of an object 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 Diatom 

 in the midst of a slide-full) which we wish to submit to high ampli- 

 fication. An arrangement for this purpose has been already noticed 

 in the description of Collins's " Harley Binocular" (Fig. 39) ; but 

 the one more commonly in use is 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 the original form of 



