ACCESSORIES. 



113 



be selected and brought into focus with 

 the low power ; then the diaphragm 

 should be opened as wide as possible 

 and later gradually closed; it will now 

 be seen that as the diaphragm is closed 

 the outlines of the object become more 

 distinct, until a point is reached when 

 the light is so much diminished that the 

 object is no more visible. It is a lesson 

 of experience to make the exact compro- 

 mise between the amount of definition 

 and the amount of illumination. 



Accessories. 



Stage micrometer. 



Ocular micrometer. 



Camera lucida. 



In the laboratory quantitative estima- 

 tions are desirable, and to ascertain the 

 size, as well as the shape of objects 

 studied is a necessity. To this end the 

 microscopist makes use of instruments 

 for measuring. The most useful is the 

 Bye-Piece Micrometer. 



This generally consists of a circle of 

 glass, accurately ruled, which slips into 

 the eye-piece and rests upon a dia- 



phrag-m. The accompanying figure il- 

 lustrates this accessory. 



The eye-piece micrometer must be 

 standardized before using, by means of 

 a Stage Micrometer, which generally is 

 made upon a glass slide. In the centre 



of the glass slide there is a finely ruled 

 scale, usually of one millimeter divided 

 into 100 parts. 



This serves as a standard, and the 

 eye-piece micrometer is measured in the 

 following way: With a given eye-piece, 



compared with the divisions of the stage 

 micrometer. It is best to take the en- 

 tire 50 divisions of the eye-piece mi- 

 crometer and compare their length with 

 the divisions of the stage micrometer. 



If these 50 divisions are found to 

 measure 7 divisions on the stage mi- 

 crometer, then 50 lines on the eye-piece 

 micrometer equal 7 on the stage mi- 

 crometer. Each division of the stage 

 micrometer equals Vioo of a millimeter, 

 hence the 50 lines of the stage microm- 

 eter equal Vjoo of a millimeter, and each 

 division of the eye-piece micrometer 



equals Vso of y^o., or "/ 



DOOOf 



or "A 



DOOO 



Fig. 7, Eye Piece Micrometer. 



,0014 mm. = IVio mikrous, or micro- 

 millimeters. 



Having standardized the eye-piece mi- 

 crometer for both eye-pieces and both 

 objectives, and entered a note of the 

 vfilues on a card for reference, it can 

 then be used to measure any object, as, 



for instance, starch grains, or oil glob- 

 ules, or resin masses. 



If the pharmacognocist is to do any 



research work, accurate delineations of 



the object seen are requisite; for this 



purpose some drawing apparatus will be 



Fig. 8. A feiimpTe Form of Abbe Camera Lucida 



say 2 inches, and a given objective, Ve, 

 and a standard tube length, 160 milli- 

 meters, the stage micrometer is brought 

 into focus. The eye-piece micrometer is 

 then introduced and its divisions are 



found of service. The w^ell-kuown Abbe 

 camera lucida is the best instrument in 

 the market at the present time for this 

 purpose, though other excellent instru- 

 ments are to be had for a much smaller 

 price. 



