OF THE MICROSCOPE. 103 



"With the best illumination, the average human eye can just 

 clearly distinguish lines which are the two-hundredth of an inch 

 apart. Some eyes, under favorable circumstances, can see 

 lines placed as close together as 250 to the inch, but the aver- 

 age is as we have stated.* To be visible even to the best eyes, 

 therefore, the lines on the Angulatum, must be magnified so 

 that they will present the same appearance as lines spaced so as 

 to give at the very most, say, 200 to the inch. This requires a 

 magnifying power of 250 diameters, and with less than this 

 they cannot be seen, no matter how good the objective may be. 

 And when Dr. Frey says that they can be seen with a power of 

 80 or 100 times, while "weaker objectives, magnifying 40 or 50 

 times, should show something of the lines, " he makes a state- 

 ment that we cannot accept. 



In order, therefore, that an object may be distinctly seen, it 

 must be magnified to a certain extent, but the magnifying 

 power absolutely necessary in any given case, will also depend 

 upon whether the microscope is to be used for general purposes 

 of investigation, or merely for the recognition of known forms. 

 For the latter purpose a power of 100 may be sufficient, while 

 for the former, on the same class of objects, a power of 500 

 would be the least that would be serviceable. The following 

 are a few of the cases in which the power required can be stated 

 approximately : 



For medical purposes (except for pocket instruments, intended 

 merely to enable the observer to recognize known forms) a 

 power of 400 is needed, and the objective should be of really 

 excellent quality. 



Students of histology require a microscope with a wider 

 range of power. Low powers are more useful to them than 

 to the medical man, and if they push their researches in cer- 

 tain directions, there is no limit to the magnifying power needed. 



*To test the statement in the text, place a glass micrometer, ruled 200 

 lines to the inch, on the stage of a microscope, and by means of the 

 mirror throw a beam of light upon it, just as if for examination by trans- 

 mitted light in the usual way. If we now look at the lines, not through 

 the tube, but simply from one side, they will appear distinctly as well-de- 

 fined lines. Try the same with a micrometer ruled 250 to the inch; some 

 eyes will be able to distinguish the lines, but very many will fail to do so. 



