DARK GROUND ILLUMINATION 139 



reflected, continues its course in a straight line, and is taken up by^ 

 the objective, as is represented by the dotted line k v'. Hence we 

 see that the same rays which are unable to enter a dry objective 

 are admitted by an oil-immersion one, and that an oil-immersion 

 lens can take up rays which fall on the slide at a very oblique angle. 



In order that these oblique rays may be present, ready to be 

 taken up by the oil-immersion objective, it is necessary to employ 

 a sub-stage condenser. It is only by means of a sub-stage condenser 

 that a " wide -angled cone of rays," as it is termed, is obtained. 

 Hence to make full use of an oil-immersion objective to " get 

 most out of it " it is absolutely essential to employ a sub-stage 

 condenser, and for the finest work a special " oil -immersion con- 

 denser " is employed. It will be obvious also that although a 

 water-immersion objective admits more rays than a dry one, it does 

 not admit so many as an oil-immersion. It must be pointed out, 

 however, that Canada balsam, or some medium having the same or 

 a higher refractive index, must be used for mounting to obtain the 

 full advantage of the oil-immersion system. The oil-immersion 

 can of course be used for examining objects mounted in water, etc., 

 cedar oil being still used between the cover-glass and the lens. It 

 is to be noted that a dry objective cannot be used as an immersion 

 one, nor an immersion objective dry, as the construction differs in 

 the two cases. 



Of late " dark ground illumination " has been much employed, 

 particularly for the examination of living objects. In this special 

 condensers are used, the central rays passing through which are 

 " stopped out," so that the object is illuminated only by very 

 oblique rays and appears white on a dark background. A dry lens 

 is used, or if an oil-immersion one, a stop must be introduced to 

 reduce its aperture, and slides and cover-glasses of special thickness 

 together with brilliant illumination are necessary. 



The lenses in the objective are formed by cementing 

 together different kinds of glass in order to correct for 

 " spherical " and for " chromatic " aberration. The rays 

 passing through the margin and the centre of a simple 

 lens are not focussed at the same point, and a distorted 

 image is the result ; this is known as " spherical aberra- 

 tion," while the violet and red ends of the spectrum, 

 being of different refrangibility, and a simple lens acting 

 like a prism, coloured fringes are observed ; this is termed 



