798 Botanical Microtechnique 



ocular scale, only five of which are shown in Fig. 16.4, are equivalent to 

 6.8 large divisions or 68 small divisions of the stage micrometer scale. 

 The computation is: 



50 eyepiece divisions = 0.68 mm. 

 1 eyepiece division = 0.0136 mm. = 13.6 microns (m) 



The curved spore in Fig. 16.4, occupies one space on the ocular 

 scale, and is 13.6 \^i long. 



The loose eyepiece disks described above are easily lost if they are 

 not kept permanently in the ocular. In a large department it is an 

 economy, over a period of years, to buy special micrometer eyepieces 

 instead of disks. These eyepieces have a built-in disk, and the eye lens 

 is adjustable to focus the scale sharply for the eyes of different indi- 

 viduals. Consult the catalogues for descriptions of micrometric 

 devices. 



Microprojectlon 



The discussion of image formation showed that an image is 

 produced if an intercepting screen is placed above the eyepoint of the 

 ocular. With a sufficiently darkened room, a brilliant light source 

 such as an arc lamp, and a good screen, an acceptable image can be 

 obtained with the highest powers of the microscope. However, the 

 most satisfactory results are usually at low and moderate magnifi- 

 cations. An image can be projected on drawing paper and a diagram- 

 matic or detailed drawing made with considerable accuracy. 

 Calibrations must be made for each lens combination and projection 

 distance. This is done by projecting the image of a stage micrometer 

 on the screen, measuring this image Avith an accurate ruler, and 

 computing the magnification. 



The catalogues and service leaflets of the manufacturers furnish 

 detailed descriptions of a -wide range of typos and price classes of 

 microprojectors. 



Types of Microscopes 



In the foregoing discussion of the elements of microscopy, the 

 various types and makes of microscopes were not specifically discussed. 

 A simple microscope is one that uses only one lens unit to magnify the 

 object. The lens unit may be a single lens. A pair of lenses in fixed 

 relation to each other comprise a doublet; a triplet consists of three 

 lenses in a mounting. The most useful magnifications range from 6 



