LENSES. 19 



upper is called the ocular lens, the lower the collective lens. The 

 objective system, which is a combination of several lenses or lens 

 systems, the lowest and smallest of which is known as the front 

 lens, is screwed into the lower opening of the tube. 



All larger instruments possess several oculars and objec- 

 tives, which together give different magnifications according to the 

 combinations used. For most objects a magnification of 500 diam- 

 eters is all that is required, but to obtain this and still have a 

 clear and bright field the ordinary lenses are hardly sufficient. The 

 greater the magnification, the darker is the field. To avoid this, 

 illuminating mechanisms (condensers, Abbe's apparatus) have been 

 constructed, by means of which the rays of light are concentrated 

 and controlled. This arrangement is absolutely necessary for deli- 

 cate work. 



Even with the aid of such an apparatus the dry objective sys- 

 tems are not sufficient. With them the rays of light must pass 

 through different media having various indices of refraction. The 

 rays pass from the object through the cover-slip, and then through 

 the air between the latter and the objective system. They are thus 

 deflected in different directions a defect which would be avoided 

 if the rays were made to pass through a single medium. This latter 

 condition may be practically brought about by placing between the 

 objective and the cover-glass a drop of some fluid having about the 

 same refractive index as the glass. The lens is then lowered into 

 the fluid. As this invention has proved useful, so-called immersion 

 lenses have been made during recent years. There are thus twcr 

 kinds of lens systems the dry and the immersion lenses. The 

 latter are divided into two groups lenses with water and those 

 with oil immersion. As oil has a greater index of refraction than 

 water, and one more nearly approaching that of glass, the oil- 

 immersion lenses are at present the best objectives that we possess; 

 Karl Zeiss, of Jena, and other microscope makers, have in late 

 years made lenses from a special sort of glass which reduces to a 

 minimum the chromatic and spheric aberration of the rays of light 

 in their passage through the objective (apochromatic lenses). 



The rays of light reflected from the mirror and passing 

 through the object are refracted by the objective system in such a 

 way that they are focused in a so-called real image at a point about 

 half-way up the tube. This picture is an inverted one, the right 

 side of the microscopic field being at the left of the real image, and 

 the upper portion below. The picture is, in other words, rotated 

 1 80 degrees. By means of the ocular the real image is again mag- 

 nified virtual image but no longer inverted, although to the eye 

 of the microscopist the field actually appears inverted. To shut out 

 the rays of light, which cause a diffused picture, diaphragms are 

 sometimes introduced into the tube as well as into the ocular. (See 

 Fig. 2.) 



The objects to be examined are placed upon a glass plate 



