38 



i 



HE MICROSCOPE 



Bull's-eye 

 condenser. 



Increase in 

 illumination, 



Fio. 26.— No. 3215, 

 Bull's-eye Con- 

 denser on Stand. 



position where it actually reaches exactly that of the glass an 

 extremely fine line shows on looking at the surface obliquely, 

 indicating that only at that exact point where the density of the 

 fluid and the glass are absolutely the same is the grey efEect pro- 

 duced by reflection destroyed and turned into black. 



A bull's-eye lens on a stand used in conjunction with a lamp, 

 or attached to the lamp itself, is required for the illumination of 

 opaque objects. It is also useful for in- 

 creasing the illumination with a dark- 

 ground illuminator or for obtaining a 

 moderately convergent beam of light in 

 combination with the mirror when a sub- 

 stage condenser is not used. It may be 

 used in connection with a substage condenser 

 for increasing the size of the image of the 

 source of light. This is of service when a 

 very small luminous source of light, such as 

 the electric " Pointolite," is employed. 



On an optical bench a condenser will 

 transfer the light source to a position nearer 

 the microscope. Except for these purposes, 

 it should not be employed with a good , 

 substage condenser. It does not give! 

 additional light, and it generally ruins the performance of a I 

 perfectly made condenser due to its own lack of correction. \ 

 Its action in increasing the illumination of opaque objects can 

 be explained by Fig. 27. A bull's-eye lens placed between the 

 source of light (S) and the object (0) produces a small image of 

 the source of light at (0). The upper diagram in Fig. 27 shows 

 the lens (L) forming a somewhat reduced picture of the source of 

 light. The lower diagram shows the distances so arranged that 

 it is forming a much reduced image. The size of the image will 

 depend on the relative distances from the light source to the 

 lens and the image to the lens. In the upper diagram L is 

 half L S, and the image is half the size of the luminous source ; 

 in the lower diagram L is one- quarter L S, and the image is 

 one-quarter the size of the luminous source. The lens does not 

 only collect a much larger amount of light than would other- 

 wise reach the 

 object at 0, but 

 it also compresses 

 it into a very 

 small size image, 

 especially in the 

 case of the lower 

 diagram^ and 

 therefore it pro- 

 duces a very con- 



FiG. 27. 



