122 PRACTICAL 'DIRECTIONS 



Suppose for example that three divisions of the micrometer occupy fifteen 

 divisions on the millimetre scale, then the magnification is = 100 x -V 5 =500. 



This is a simple and convenient method but the results are only approxi- 

 mate, the magnification being somewhat exaggerated. 



B. With the ocular micrometer. 1 An ocular micrometer consists of a small 

 circle of glass on which a scale divided into mm. is engraved. The ocular 

 micrometer is placed between the eye and field lenses of the eyepiece. 



The magnification of the eyepiece being known (generally 10 diameters), 

 each division of the scale as seen through the eyepiece is equal to -^ x 10 

 mm. = 1 mm. 



1. Place the stage micrometer on the stage of the microscope, drop the 

 ocular micrometer into the eyepiece, and turn on the objective to be examined. 

 Adjust the tube to the proper working distance and focus the scale on the 

 stage micrometer, then arrange the latter so that any two lines on it coincide 

 with any two lines on the ocular micrometer. 



2. Determine how many divisions of the ocular micrometer are covered 

 by one division of the stage micrometer, and let n be the number. 



The magnification produced is given by 



and if five divisions of the ocular micrometer are covered by one division 

 of the stage micrometer, the magnification is =5 x 100. 



2. The measurements of objects under the microscope. 



The standard adopted for microscopical measurements is the one-thousandth 

 part of a millimetre, which is designated by the Greek letter u ; the Bacillus 

 tuberculosis for example is said to measure T7 to 3'5/* long by 0*2 to 0'5/x 

 broad. 



Two different methods may be employed for measuring microscopical 

 objects. 



A. Camera lucida method. 1. First ascertain the magnifying power of 

 the system of lenses to be used by means of the objective micrometer and 

 camera lucida (p. 121). 



2. Substitute the slide on which the object to be measured is mounted for 

 the stage micrometer, and an outline of the object will be thrown on a sheet 

 of paper arranged as for the preceding determination. 



3. Measure the length of the outline in millimetres, and let n be the length. 



4. Then, the magnifying power m of the combination of lenses being known, 

 the diameter D of the object is easily determined from the equation 



Let us suppose the magnifying power of the optical system to be 500 diameters, 



[* Hermann, Whittaker, and Young warn against the use of an Huygenian eyepiece 

 with a micrometer. Ramsden's is the only eyepiece that can be relied upon. Clearly a 

 micrometer scale put between the lenses of an Huygenian eyepiece will only be magnified 

 by the eye lens and will therefore undergo "pincushion" distortion. But the image of 

 the stage micrometer at that place has "barrel-shaped" distortion which is rectified by 

 the "pincushion" distortion produced by the eye lens. 



[The object must be in the centre of the field, as towards the periphery " pincushion " 

 distortion would be more marked. 



[Using the same microscope measurements made with the same combination of lenses 

 are comparable among themselves but are not comparable with measurements made 

 with any other microscope nor with the same microscope and any other combination of 

 lenses.] 



