24 



the rays when viewing opaque objects by reflected light ; for photo- 

 micrography it is indispensable. It should have a ball and socket 

 movement. 



Dark-ground illumination of opaque objects is effected in various 

 ways, the simplest being by means of the bull's eye condenser, pre- 

 senting the plane side of this close to the object ; but a better effect 

 is obtained by using the latter in conjunction with the side silver 

 reflector. With their use the objects require to be specially mounted 

 in a black cell. The " Lieberkuhn " is a useful device for this pur- 

 pose, but can only be used with the lower powers, one being required 

 for each objective. The central spot diaphragms, the " spot " lens, 

 and the paraboloid are other means of obtaining dark-ground effects ; 

 but the two latter are not now much used. Dark-ground illumina- 

 tion with high powers is effected with the "vertical illuminator," a 

 piece of apparatus fitted just above the objective ; it is the means 

 employed in examining the surface of metals, minerals, etc. Very 

 beautiful effects are obtained by the use of polarised light, but an 

 explanation of its theory would take more time than can be allotted 

 to it here. For the study of crystallography and micro-chemical 

 analysis a knowledge of its use and properties is essential. 



The camera lucida is a valuable addition to the microscope for 

 making drawings of objects, etc., and for obtaining measurements 

 with the micrometer. It is an apparatus properly mounted and fitted 

 to the eyepiece of the microscope, which latter is inclined to the 

 horizontal position ; the image is then projected at right angles on 

 a piece of white paper and the outline traced with a pencil. Photo- 

 micrography is, however, now superseding this method of reproducing 

 the microscopic image. 



For the accurate measurement of objects one method is by means 

 of a stage micrometer. This is a glass slide ruled in y^ths and 

 ToVoths of an inch, or y^ths and y^-ths of a millimetre, and costing 

 five shillings. The object is focussed on a screen as in photo- 

 micrography, note being taken of the exact position of the projected 

 image. It is then replaced by the stage micrometer, and the corre- 

 sponding measurement of the image of its divisions taken. Another 

 method, and one generally adopted, is by using in addition to the 

 stage micrometer an eyepiece micrometer. This is a glass disc ruled 

 with arbitrary divisions and is placed upon the diaphragm of the eye- 

 piece ; the number of divisions corresponding with the object is then 

 noted and read off on the stage micrometer, which has meanwhile 

 replaced the object. This method entails the purchase of an eye- 

 piece micrometer costing five shillings. The " Ramsden ; ' eyepiece 

 micrometer is probably the most convenient of all methods. It is, 

 however, somewhat costly. For the measurement of the thickness 

 of an object some microscopes have the fine adjustment screw so 

 graduated that each turn of the screw represents a proportionate 

 measurement in millimetres. 



It follows, of course, that given a photograph or drawing of an 



