2»Ji8 Journal of Applied Microscopy 



are cemented in the mounts. Microscopes should not be kept exposed in rooms 

 where acid fumes are much liberated. 



A few accessories for use with the microscope should be mentioned at this 

 time, since in some kinds of investigations they are very important. 



The Polarizing Apparatus. This consists essentially of two Nicol's prisms, 

 one of which is placed below and the other above the object. The one below 

 the specimen is called the polarizer and serves to polarize the light before it 

 reaches the object. It is usually mounted in some form of collar so that it can 

 be supported in the substage apparatus between the mirror and condenser. 



The Nicol used above the object is called the analyzer. It is usually placed 

 in one of three different positions, viz.: (1) it is fitted to set as a cap on top of 

 the eyepiece mount ; (2) it is placed in a special form of eyepiece between the 

 two combinations of lenses, or (3) it is supported in some way in the barrel of 

 the microscope between the objective and ocular. The last mentioned position 

 is the common arrangement for the petrographical type of instrument. 



Polarized light is especially useful in the examination of certain crystals, 

 fibers and starches. In using the polarizing apparatus the substage diaphragm 

 is opened wider than in ordinary light, since in the process of polarizing the 

 light about half of the luminous effect is lost. The object to be examined is put 

 in place and brought into view with the polarizer in position. The analyzer is then 

 put in place and rotated around the vertical axis until the point is reached where 

 the field appears darkest. In this condition, objects which are doubly refractive 

 display colors to a greater or less degree. The intensity and shade of color 

 change with changes in the relation of the object to the planes of polarization. 



In order to increase the effect produced by substances that are only slightly 

 doubly refractive a selenite disk should be employed. The disk giving red of 

 the first order is the most useful for this purpose. It can usually be secured 

 from the maker of the instrument, to fit within the mount of the polarizer. It 

 produces a red field in the microscope in which doubly refractive objects appear 

 either in some shade of blue or purple when in one position and in shades of 

 orange or yellow when rotated through 90°. The contrast thus produced not 

 infrequently constitutes an important factor in determining the nature of 

 objects. 



The eyepiece micrometer is a fine micrometer scale engraved upon a circular 

 plate of glass which can be placed upon the diaphragm of the eyepiece by un- 

 screwing the upper lens mount or else by means of the divisible eyepiece tube 

 furnished with part of the compensating series. It is used in making micro- 

 scopical measurements. The value of each of the scale divisions M'ill depend 

 upon the objective, ocular and tube length ; and hence must be determined for 

 each combination used. In determining this value, which we may call the micro- 

 meter modulus, and ever after when using it, the tube length must of course be 

 that for which the objectives are calculated. This length is most frequently 

 measured from the upper end of the draw tube where the ocular is inserted to 

 the lower end of the barrel where the objective is screwed on. 



To determine the modulus accurately a stage micrometer is required having 

 a scale marked in divisions of .01 mm. (or else in fractions of an inch). If the 



