MEASUREMENT. 



[ 417 ] 



MEASUREMENT. 



scale graduated into inches, tenths, and 

 hundredths. 



The simplest method is that by double 

 sight, as it is called. The micrometer slide 

 is placed upon the stage, the lines brought 

 into focus, and the image of one of the in- 

 terspaces, as seen upon the stage with the 

 open eye not used in looking through the 

 microscope, is measured with compasses. 

 By then dividing the measure of the image 

 of the space by the known measure of the 

 unmagnified space, the quotient is the re- 

 quired magnifying power. Thus, if the space 

 on the micrometer scale is equal to the 

 1- 100th of an inch, and the image of the 

 magnified spact corresponds to 5-10ths of 

 an inch, the space is magnified 50 times : 

 A-s-T^rrSO. 



The same result may be obtained with the 

 aid of the camera lucida, by placing the 

 microscope horizontally, and its axis at a 

 distance from the table equal to the distance 

 between the focus of the eyepiece and the 

 stage ; the breadth of the image of a division 

 is then measured as before ; and this is the 

 best and most certain method. 



The most important point in relation to 

 this subject is, that the joint of the micro- 

 scope shall be furnished with a stop or pin 

 (!NTR. p. xiii), by which the body may be 

 placed horizontally at once, so that all ob- 

 jects which are drawn under the same object- 

 glass and eyepiece may be magnified to the 

 same extent ; and this" should be determined 

 by the second of the above methods. 



The obvious use of being acquainted with 

 the magnifying power of a microscope, is 

 that objects under examination may be 

 viewed by the same power as that with 

 which figures of them have been made, so 

 that the structure or appearance of the ob- 

 jects in the two cases may be compared. 

 We have elsewhere stated the importance of 

 expressing the magnifying power with which 

 figures of objects have been drawn (!NTR. 

 p. xxxix). 



In the above estimation of the magnifying 

 power, one dimension only is taken into 

 account, viz. the breadth or diameter ; and 

 this is the ordinary manner in which the 

 magnifying power is taken ; objects are then 

 said to be magnified so many diameters, or 

 so many times linear. But objects are really 

 as much magnified in the other dimension, 

 or in their entire surface ; so that the true 

 expression of the amplification would be 

 given by multiplying that in one direction 

 by that in the other, or by itself, i. e. squaring 



the linear magnifying power. This is called 

 the superficial measurement. 



This proceeding, however, offers no advan- 

 tage, and is not in accordance with custom, 

 either in regard to the microscope or objects 

 in general. It is therefore never used except 

 for fraudulent purposes, to delude the un- 

 wary in the purchase of an instrument ; thus 

 supposing a microscope to magnify 40 dia- 

 meters, 40 X 40 = 1600 would express the 

 magnifying power in superficial measure. 



Measurement of the size of objects. This 

 is effected with the aid of a slide-micrometer 

 passed through two slits in the eyepiece 

 above the stop, and at the focus of the upper 

 glass of the eyepiece. The breadth of the 

 spaces between the lines must be such as to 

 give an even and minute fraction of an inch. 

 The value of the spaces will vary with the 

 power of the object-glass and eyepiece, so 

 that it must be determined in each case re- 

 spectively, and recorded. For measuring 

 small objects, the breadth of the spaces in 

 the eyepiece micrometer may be such that 

 twenty of them correspond to 1- 1000th of an 

 inch in the stage-micrometer slide, so that 

 the value of each division will be the 

 1 -20,000th part of an inch. It is seldom that 

 we have to measure objects so small as 

 this ; but the small size is of great advan- 

 tage, because in most cases it will happen 

 that the margins of the objects will coincide 

 exactly with some of the fines, whereby the 

 chance of error in computation will be 

 avoided. For larger objects, the spaces of 

 the eyepiece micrometer may be coarser. 



The method of measuring scarcely requires 

 further explanation. Supposing, however, 

 that the divisions of the stage-micrometer 

 are equal to 1 -1000th of an inch, and those 

 of the eyepiece micrometer equal to 

 I -20,000th of an inch, i. e. twenty of them 

 cover one space in the former, an object 

 brought into focus and covering five of the 

 spaces of the eyepiece micrometer, will be 

 1 -4000th of an inch in diameter ; and so 

 for other dimensions. When the objects 

 are large, the compasses and the ivory scale 

 will suffice for their measurement; but 

 sometimes this may be conveniently done 

 under a low power, for the l-100ths of an 

 inch are not very clearly discernible to all 

 eyes. 



In measuring objects, they must be co- 

 vered with thin glass, and not immersed in 

 too much liquid. 



It is a matter of great difficulty, under 

 high powers, to adjust accurately the divi- 



2E 



