MATONIA. 



C 493 ] 



MEASUREMENT. 



M. actt7iopti/chus (PI. 18. fig. 23 b) the 



angles and rays are nine in one valve, and 

 thirteen in the other, and so on. Diameter 

 from 1-1600 to 1-300". 



M. he.va<iona (PI. 18. %. 25). 

 BiBL. Ehrenb. Bet: Bed. Akad. 1844; 

 Kiitz. Sp. Alg. 25. 



MATO'NLl, R. Brown.— A genus of 

 Cvathese (Polypodiaceous Ferns). 



M. pectinata (figs. 452-454) Exotic. 

 (Hook, Si/n. 45.) 



_ MAUliAN'D YA.— A genus of Scrophula- 

 riacese (Dicotyledons), the testa of the seed 

 of which is composed of cells with spiral- 

 fibrous deposits, foradng an elegant micro- 

 scopic object. 



MEASUREMENT and Measures.— In 

 this article we shall consider the method 

 of measuring the magnifjang power of a 

 microscope, of ascertaining the dimensions 

 of objects, and shall give a sketch of the 

 standard measures in which the dimensions 

 of objects are expressed. 



Measurement of the magnifying power of 

 a microscope. — The apparent size which an 

 object appears to possess under a micro- 

 scope will vary according to the power of 

 the object-glass and of the eye-piece used, 

 and the length of the body of the micro- 

 scope ; and it is a good plan to determine 

 the measurements once for all in the case 

 of the various object-glasses and eye-pieces, 

 keeping them wi-itten upon a card, so that 

 they may be readily accessible. 



The apparatus requisite consists of a 

 glass micrometer-slide graduated into thou- 

 sandths of an inch, each tenth division being 

 marked by a longer line ; or two separate 

 slides, one graduated into thousandths, the 

 other into hundredths of an inch ; and an 

 ivory 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 

 interspaces, 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 

 unmagnitied space, the quotient is the 

 required magnifying power. Thus, if the 

 space on the micrometer-scale is equal to 

 the 1-lOOth of an inch, and the image of 

 the magnified space corresponds to S-lOths 

 of an inch, the space is magnified 50 times : 



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. 



A most important point in relation to 

 this subject is that the joint of the micro- 

 scope shall be furnished with a stop or pin 

 (Introduction, p. xv), by which the body 

 may be placed horizontally at once, so that 

 all objects which are drawn under the 

 same object-glass and eyepiece may be 

 magnified to the same extent, the degree 

 being 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 

 objects in the two cases may be compared. 

 In the above estimation of the magnifying 

 power, one dimension only is taken into 

 accoimt, viz. the breadth or diameter; and 

 this is the ordinary manner in which the 

 magnifying power is stated ; objects are 

 then said to be magnified so many dia- 

 meters, or so many times linear. 



Measurement of the size of objects. — This 

 is efi'ected 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, 

 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-lOOOth of an inch 

 in the stage-micrometer slide, so that the 

 value of each division wiU be the l-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 advantage, because in 

 most cases it will happen that the margins 

 of the objects will coincide exactly with 

 some of the lines, 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 



