THE OPTICAL EQUIPMENT 19 



certain aberrations inherent in an objective. These may be for 

 convenience again subdivided into low- and high-power lenses. 

 The low powers are always what are known as dry lenses, and 

 with them a magnification of not more than five hundred 

 diameters would be obtainable. The high powers are chiefly 

 what are known as immersion lenses ; that is, between the object 

 and the front lens of the objective an immersion fluid sometimes 

 water, but usually cedar-wood oil is used ; or, in the case of some 

 lenses of very high aperture (to be obtained from Messrs. Zeiss), 

 monobromide of naphthaline. The object of the immersion 

 fluid is to enable a wider beam of light to be utilised by the 

 objective. Owing to cedar-wood oil having a refractive index 

 approximately the same as glass, the system becomes what is 

 known as ' homogeneous ' ; that is, assuming the object to be 

 mounted in contact with the cover-glass, or in a medium of 

 nearly the same refractivity as glass, little or no refraction of the 

 light occurs from its leaving the object, until it passes through 

 the objective. Consequently, more light is transmitted to the 

 objective, and a beam of wider angle can be utilised. 



Fig. 6 represents diagrammatically a section through the 

 cover-glass covering any object and the front lens of an objective. 

 The left half represents the conditions under which an oil- 

 immersion objective is used, and the right half shows the 

 air-gap between cover -glass and objective which exists with 

 dry lenses. 



It will be seen that the ray AB undergoes refraction at B 

 when entering the cover -glass, and it is therefore refracted to- 

 wards the normal BN, and, on leaving the glass, is again refracted 

 in the direction DA' parallel to AB. The consequence is that 

 the ray AB issues from glass into air as DA', and does not enter 

 the front lens of the objective. The ray CBC' is the ray farthest 

 from the normal that enters the lens. The condition of affairs 

 when oil is interposed is seen on the other side, where the 

 ray EB is again refracted by the cover -glass ; but, owing to the 

 oil being of practically the same refractive index as the glass, the 

 ray continues in the same straight line in the direction BE' and 

 enters the lens, so that it goes to take part in the formation of 

 the image. A ray, therefore, such as FB, which is very much 

 more oblique and which would take the direction BF' on refrac- 

 tion through the glass, would just enter the lens. It is evident, 



o2 



