THE MICROSCOPE 69 



a certain delicacy of movement to put a specimen in a 

 desired position. 



10. Beginners should always use the low-power objectives 

 and oculars first. The low-power objectives have longer 

 working distances and always show a larger portion of the 

 specimen. After obtaining a general idea of the specimen, 

 desired portions may be examined with the higher power 

 objectives. 



11. In using high-power objectives for finding and exam- 

 ining a specimen, it is always more desirable to use the low- 

 est power ocular (corresponding to Leitz No. 1). If a higher 

 ocular is used, there is a loss in the depth or sharpness and 

 size of field, since they are both inversely proportional to 

 the magnification. Illumination is also lost, which varies 

 inversely as the square of the magnification. Remember 

 that the largest field, the greatest penetration, and the best 

 illumination are obtained by using the lowest magnification 

 which makes all the detail in the image visible. 



Oil Immersion Objective. The highest power objective 

 is the oil immersion lens. This is so termed because a drop 

 of oil must be used between the front lens and the cover- 

 glass. The oil used must have the same index of refrac- 

 tion as glass to prevent the dispersion of the rays of light 

 coming from the condenser. 



Working distance is the free distance between the cover- 

 glass and the objective when the latter is focused. High- 

 power objectives have short working distances. 



REFERENCE 



GAGE: The Microscope. 



