rii()H)MicR(>c.i<.iriiy .ixn riiiiMCii. MicRoscnrv i.?j 



nii.mtitativf in\fsti>;alii)ns an- in.idi- 1>\' CDimtiii^ ilu- visil)le 

 partirli-s in a givon volume- of tlie tluid and ihc nianniT in wliirh so 

 novel an in\t'stigatii>n ran Ir' acconiplislu-d by optical int-ans >^liuuUI 

 prove of general interest. One nietliod consists in the use of the 

 f\-epiece niicroineter which is substituted for the ordinary eyepiece 

 of the observing microscope. The eyepiece micrometer is ruled into 

 stjuares and the dimensions of these are found by calibration with a 

 stage micrometer. The depth of the stratum is measured by turning 

 the slit head through a right angle and thus a solitl is blocked out in 

 till- path of the light rays, whose length and breadth are defined by 

 the rectangular area of the niicrnmcter e\epiece and whose depth is 



Vig. 2i — Illustrating the adaptation of micrometry to the ultra-microsco|)e for the 

 purpose of counting particles per unit volume. 



that of the light beam and may be read from the known dimensions 

 of the eyepiece micrometer. Fig. 23 shows the cross ruling of the 

 eyepiece and the pencil of light which traverses the field. The length 

 of the side of each square as seen through the water immersion objec- 

 tive with a tube length of 160 mm. has an approximate value of 9 ft 

 as referred to the object, which value is sufficiently accurate for 

 ordinary measurements. Where more exact measurements are 

 requireii, the ruling is calibrated for the eyepiece and objective by 

 means of a stage micrometer in the manner to be described under 

 the subject of micrometry. 



For studying the behavior of particles in polarized light the eyepiece 

 is fitted with an analyser. In a measure, as the particles decrease 

 in size they become more strongly polarized in degree towards the 

 plane which passes through the axis of the illuminating and diffracted 

 rays, i.e., the principal plane of difTraction. The analyser also serves 

 to distinguish unpolarized from polarized light. 



The apparatus for examination of solids is identical in so far as 

 the illuminating train is concerned with the apparatus for liquids 

 just described. It differs only in the character of the specimen or 

 of the cell used and, while flesigned primarily for transparent solids, 

 it may be used with a suitable cell for licjuids also. When liquids 

 are being examined, there is no need for searching the specimen since 



