REQUISITES OF A GOOD MICROSCOPE. 27 



is the best form in the market at the present time. In drawing, 

 the microscope must be erect and the paper horizontal and at 

 the distance of distinct vision, about 12 inches. 



DETERMINATION OF MAGNIFICATION IN A MICROSCOPE. The 

 magnifying power for certain combinations of objectives and 

 eye-pieces and tube length is usually stated by the makers, so 

 that it is hardly necessary now to determine the magnifying 

 power. But sometimes the rating of the makers is not correct 

 and we might want to use a different tube length, and, again, 

 the distance of most distinct vision for our eyes might not be 

 the average distance, namely, 12-14 inches, in which case a new 

 determination must be made, which may be done in the follow- 

 ing manner : 



A STAGE MICROMETER a piece of glass, accurately ruled to 

 hundredths of a millimeter is placed on the stage and brought 

 into focus. By means of a camera lucida, the magnified scale 

 and an accurate mm. scale, placed at the distance of distinct 

 vision alongside the microscope and parallel with the mi- 

 crometer scale, are brought into superposition. The number of 

 mm. divisions covered by a definite number of the micrometer 

 scale divisions is then noted. Suppose each magnified scale 

 division covers 5 mm. of the rule, what is the magnifying 

 power? One mm. is equal to 100 micrometer divisions, 5 mm. 

 covered by one micrometer space are equal to 500 micrometer 

 divisions. Hence one micrometer division has been magnified 

 so as to cover a space 500 times as wide, i. e., it has been mag- 

 nified 500 times. This, then, is the power of the instrument for 

 the particular combination. 



SOURCE OF LIGHT. The best source of light is a white cloud 

 or the diffused light reflected from a white wall or other white 

 object. Never use direct sunlight. Light from the blue sky is 

 not so good as that from a white surface. There is a tendency 

 among beginners to use the strongest light possible. This is 

 injurious to the eyes and often obscures details of the object by 

 its dazzling glare. A window facing north is best. 



REQUISITES OF A GOOD MICROSCOPE. It goes without saying 

 that the best workmanship must be found in the mechanical 

 parts. The foot, pillar, arm, stage, etc., must be of sufficient 

 weight and strength and size. For the optical parts five points 

 must be considered : 



1. WORKING DISTANCE. This is the distance between the 

 front lens of the objective and the object. The lower the mag- 

 nifying power the larger the working distance in general. 

 Working distance has no fixed relation to the focal length, but 

 varies with the mode of construction and the aperture of the 

 objective. Of two objectives having the same focal length, that 

 one with the larger working distance is to be chosen. As the 



