202 THEORY OF MICROSCOPIC OBSERVATION. 



If we lower the focus of the Microscope to the level of the focal 

 point, the images of distant objects reflected in the mirror, or which 

 are within the incident cone of light, are seen. The outlines are 

 the more sharply defined the more we exclude rays incident 

 obliquely to the axis by the arrangement of the diaphragm or of 

 the illumination, and the less the marginal rays of the cone of 

 light reaching the objective deviate from the perpendicular. A 

 parallel pencil of light incident vertically from below gives, with a 

 moderate amplification, such a sharp image, that it immediately 

 becomes indistinct on somewhat higher or lower focal adjustment. 



The unequal refrangibility of the differently-coloured rays causes 

 the emergent cone to receive only red rays in the centre and violet 

 at the periphery. We might hence be tempted to explain the fact 

 that the bright colourless circle, which the image of the diaphragm 

 presents, is, on slightly higher focal adjustment, not only consider- 

 ably larger, but also shows a reddish centre and a bluish fringe. 

 These colour-phenomena, as with real images, do not originate in 

 the object itself, but are due to the chromatic aberration of the 

 Microscope, which is over-corrected by the higher adjustment and 

 under-corrected by the lower. This is demonstrated by the 

 simple experiment, which is usually employed in testing the 

 aberration, viz., by viewing the image of a window reflected in a 

 globule of mercury. Eaising or lowering the focus produces, 

 therefore, the same colour-phenomena as those exhibited by the 

 images of air-bubbles formed by refraction. 



2. GLOBULES OF OIL IN WATER. 



We will consider globules of oil in water as representing in 

 general a circular vertical section of any given object immersed 

 in a medium of lower refractive index. In order to determine 



limiting inclination is found to be about 18, if the refractive index of the glass 

 is 1 '5. This inclination determines also the position of the focal point of an 

 air-bubble. It produces, moreover (if the objective is not immersed in water), 

 a further diminution of the focal length, which is due to the fact that an object 

 in water appears the higher the deeper it lies. The focal point is therefore 

 more raised than the centre of the air-bubble, and is hence apparently moved 

 nearer to the latter. This phenomenon, which must not be neglected in direct 

 measurements of differences of level, will be fully discussed later on. 



