60 Boss's Four-Tenths Condenser. 



tances from the angle, horizontal lines uniting the two lines 

 that converge to form the angle. The figure will be like the 

 preceding diagram, in which A represents the focal point of a 

 series of lenses, 1, 2, 3, 4, of different sizes and focal lengths. 

 Each lens, 1, 2, 3, or 4, can throw, through any transparent 

 substance, appropriately situated, rays of light which converge 

 at an angle of 100° ; but there will be a great difference in the 

 total quantity of light which each lens can receive or refract, 

 and also in the proportion which the marginal rays bear to the 

 central ones. In 1, the marginal rays will be greatly in excess 

 of the central rays ; while in 4, as the marginal rays are fewer, 

 and the central rays the same, the latter will be relatively much 

 more important. This diagram distinctly expresses to the eye 

 the fact that, when achromatic condensers are made with 

 small lenses, the marginal rays do not exceed the central rays 

 to anything like the same extent as when larger lenses are 

 employed. 



Suppose the diagram to be on such a scale, that the lens, 

 whose diameter is represented by 2, will • focus through the 

 thickness of an ordinary glass slide, it is then plain that lenses 

 of the same aperture, and so small as either 3, or 4, cannot 

 possibly perform with their whole aperture, or anything like it, 

 through the thickness in question. A lens of given aperture 

 and of focal length greater than the thickness of a glass slide, 

 may converge all its rays through any transparent object on 

 the slide ; and if the slide be much thinner than the length of 

 its focus, all the difference is, that the thin glass will obstruct 

 and refract the passage of the rays less than the thicker glass. 

 If, however, we try to make a large-angled and small lens with 

 a short focus work through a slide, the thickness of which 

 exceeds the length of its focus, only the central rays, and those 

 near them, can get through in the direction required. Thus, 

 if a microscopist desires to have a large-angled condenser, for 

 use with ordinary glass slides, as well as with thin glass, he 

 must not take an optical combination of small lenses and very 

 short focus, for if he does, a considerable portion of the slant- 

 ing light he desires should reach his object will never get there 

 at all. 



It appears to us that two things should be required of 

 achromatic condensers intended for general use and for research. 

 First, that the optical combination employed should be adapted 

 to a considerable range of power — say from an inch or two-thirds 

 upwards to the highest; and, secondly, that it should be 

 capable of woi^king with a large aperture through ordinary 

 glass slides. When low powers are employed, a pleasantly lit 

 field can be obtained, by using a condenser a little out of 

 focus, so that the rays cross before reaching the object ; 

 but a condenser for general use should be able to send 



