Objectives 59 



the brilliance of the image, and the working distance, are 

 all increased. 



It is a well-known law that rays passing from a rarer to 

 a denser medium are refracted towards the perpendicular, 

 and vice versa. If, therefore, an object be examined with 

 a dry objective, it is obvious that certain rays of light 

 emerging from the denser crown cover-glass into the rarer 

 medium, air, are refracted so far from the perpendicular as 

 to fail to assist in forming the image. By placing a medium 

 between the cover-glass and the objective, these rays are 

 utilized, owing to the influence of the dense medium, oil. 

 The refractive index of air is TO, that of water 1*33, while 

 that of cedar-wood oil is 1*52. It will be seen from this, 

 that the utility of the oil must be very appreciable, in fact, 

 an oil immersion lens receiving light at 82 and a water 

 immersion lens receiving light at 96 admit the same rays 

 as a dry lens of 180, and, therefore, divide as many lines to 

 the inch as the maximum number possible with a dry lens. 

 If immersion lenses have greater apertures than the above- 

 named they will divide finer markings than any dry lens, 

 and they can be theoretically carried to oil and water angles 

 respectively of 180. 



There is another feature of advantage gained by the 

 use of an oil immersion lens. The refraction caused by 

 the influence of the cover-glass thickness referred to on 

 page 65 does not take place, on account of the continuity 

 established between the objective and the cover-glass by 

 the immersion oil. There is, therefore, no necessity for 

 such objectives to be provided with a correction collar for 

 variations in thickness of cover-glass ; a slight correction 

 of the same kind has, however, sometimes to be made on 

 account of the distance which the object may be beyond 

 the cover-glass when the mounting medium has not the 

 same refractive index as the cover-glass. This can be 

 efficiently effected by either extending or shortening the 

 body-length. Water immersion objectives do not yield 

 so high an aperture as the oil immersions, and as the 



