CHAPTER VII. 

 THE MICROSCOPE. 



Introduction. 



Section I. The microscope stand, p. 106. 



Section II. The optical parts of the microscope, p. 107. 



A. The objectives, p. 107. B. The eyepieces, p. 116. 

 Section III. The care of the microscope, p. 117. 

 Section IV. The method of using the microscope, p. 118. 

 Section V. The measurement of microscopical objects, p. 121. 



1. The experimental determination of the magnification produced by a system of 

 lenses, p. 121. 2. The measurements of objects under the microscope, p. 122. 

 Section VI. Dark-ground illumination, p. 123. 



1. The application of dark-ground illumination to micro-biology, p. 124. 2. The 

 construction of the dark-ground illuminator, p. 124. 3. The method of using the 

 instrument, p. 125. 



FOR bacteriological work a good microscope, which will magnify from 600 to 

 1200 diameters, is necessary. It is seldom that a higher magnification than 

 1200 diameters is required, though for a few micro-organisms, e.g. the organism 

 of pleuro-pneumonia, a magnification of 2000 diameters may be useful. It 

 is to be remembered however that with the very best instruments it is 

 impossible to see organisms measuring less than 0*0001 mm. (0*1 //.) in diameter 

 (p. 113). 



A microscope may for purposes of description be regarded as consisting 

 of two parts, the mechanical (the microscope stand) and the optical (the 

 lenses) portions respectively. 



SECTION I. THE MICROSCOPE STAND. 



The stand of the microscope must be firm and rigid, and it is desirable 

 that the base be hinged to the body so that the latter can be tilted. The 

 tube should have a rack and pinion mechanism and a micrometer screw 

 adjustment for the grosser and more delicate movements respectively in 

 focussing. The stage, of ebonite or metal, should be large, and it is an advan- 

 tage if it can be centred and mechanically moved. The mirror, by means of 

 which the light is transmitted to the object, should be concave on one side 

 and flat on the other. The stand, moreover, should be so constructed that 

 an Abbe condenser can be fitted below the stage. A diaphragm either of the 

 cylindrical or iris pattern is also essential. It will be found a great advantage 

 to have a triple nosepiece capable of carrying three objectives, so that one 

 lens can be readily and quickly substituted for another. 



