USING THE MICROSCOPE 375 



In using the stage micrometer, place the cardboard on the table, 

 and with the- aid of the camera lucida sketch the rulings of the mi- 

 crometer. In Figure 139 note, for example, the scale drawn with 

 Spencer 16 mm. objective, ocular X6. The spaces are drawn from the 

 tenths of a millimeter rulings of the stage micrometer. Therefore, each 

 space on the card represents one-tenth of a millimeter or 100 n, and the 

 ten spaces shown on the card represent 1 mm., or 1,000 /x- By measur- 

 ing with a metric rule the ten spaces upon the card, it is found that the 

 scale is 102 mm. in length. The magnification of any drawing made 

 with the same ocular and objective, under the same conditions, will 

 therefore be 102 diameters. This does not mean that the magnifying 

 power is 102 diameters, for the magnification of this combination is 

 much less. A scale drawn at the level of the stage would show more 

 nearly the magnifying power of the combination, but would still give 

 too large a figure. The exact size of any object which has been sketched 

 with this combination can now be measured by applying the cardboard 

 scale, just as one would measure gross objects with a rule. 



The diameter of the field with this combination is 1,700 m- By 

 knowing the diameter of the field with the various combinations, one 

 can guess approximately the size of objects. 



Other combinations are made in the same way. An excellent check 

 on the accuracy of the computations is to measure the same object by 

 means of the ocular micrometer and by the scale card. If the results 

 are the same, the computations are correct. 



In making sketches, it is a good plan to add the data which would 

 beneeded at any time in making measurements, e.g., Spencer objec- 

 tive 16 mm., ocular X6, table, 110, 45°, would show that the sketch 

 was made at the level of the table, with the mirror bar at 110, and the 

 camera mirror at an angle of 45°. 



ARTIFICIAL LIGHT 



During a considerable part of the year daylight is often insufficient 

 for successful work with the microscope. Numerous contrivances for 

 artificial illumination have been devised, some of them fairly good, but 

 most of them thoroughly unsatisfactory. The best illuminator is the 

 one which will give the most light with the least heat. 



More than two hundred years ago Hooke used a device for artificial 

 illumination which probably suggested the apparatus used by the late 

 Professor Strasburger at Bonn. The apparatus consists, essentially, 



