8 PRACTICAL MICBOSCOPY. 



dred, etc. The instrument which I have heretofore described, with the 

 high power in use and the tube withdrawn, will present the corpuscle 

 as averaging very nearly one-eighth of an inch in diameter x 400. 

 Whil ethis gives a gross idea of amplification, the method will often 

 prove inaccurate because of individual errors in the estimation of 

 proportions. 



Use of the Stage- Micrometer. From a dealer^in optical goods pur- 

 chase a Rogers'' * glass stage-micrometer, ruled in hundredths, thou- 

 santdhs, and five-thousandths of an inch. Also procure from the 

 dealer in drawing instruments a two-inch boxwood rule divided deci- 

 mally to fiftieths. 



Place the micrometer on the stage of the microscope and focus the 

 lines. Then place the rule also on the stage, but Justin front of and 

 parallel with the micrometer. By a little practice, using both eyes, 

 the two rulings may be seen simultaneously, and by adjusting the 

 position of the rule, the lines may be made to appear superposed. 



Let us suppose that, with a given eye-piece and objective, the 

 thousandth divisions on the micrometer correspond exactly with one 

 of the tenths of the rule. Keeping this in mind, remove the micro- 

 meter scale and substitute an object, say a blood slide. Let us again 

 suppose that the image of a given red corpuscle appears to cover 

 three of the one-tenth inch rulings, the latter scale having been left 

 in position. It is evident that, as we found the value of one of the 

 rule tenths to be, by the micrometer, the one-thousandth of an inch, 

 the globule measures one three-thousandth of an inch in diameter. 



The value of the rule divisions must be determined for each objec- 

 tive ; and a memorandum will then provide the means of quickly ob- 

 taining a very close approximation of the size of objects as viewed in 

 the microscope, and at the same time indicate the degree of ampli- 

 fication of the instrument itself. 



SKETCHING FROM THE MICEOSCOPE. 



Let me most emphatically urge the practice of sketching in con- 

 nection with microscopy. " I am no artist," or " I have no skill in 

 drawing," is often the reply to my advice in this matter. I then sug- 

 gest that no special skill is needed to begin with, only patience and a 

 dogged determination to succeed. The pictures in the microscopic 

 field have no perspective, and may be reproduced in outline merely. 



* The micrometer rulings of Professor Rogers, of Cambridge University, 

 are without doubt of surpassing excellence. They are the result of many 

 years of unwearying experimentation and are recognized standards through- 

 out the scientific world. 



