124 TRANSACTIONS OF THE 



opposite form and different material, and differing from each other 

 in the proportion in which they respectively refract and disperse 

 the light so that the one medium may by equal and contrary 

 dispersion counteract the dispersion caused by the other, without, 

 at the same time, wholly neutralising its refractions. The media 

 most suitable for this purpose are the crown glass and the denser 

 flint glass. As previously stated, Frauenhofer made, in 1811, the 

 first achromatic object-glasses for the microscope, by combining 

 lenses of crown and flint glass, and Mr. Jackson Lister was the 

 first to cement concave and convex lenses of crown and flint glass 

 together by means of Canada balsam. A lens so achromatized looks 

 as if made of one piece. Sometimes the balsam becomes cloudy 

 between the surfaces, which destroys the optical properties of the 

 lens. 



I will now explain the construction of the object-glasses, the 

 number and quality of which constitute the true value of a 

 microscope. Object-glasses in England are designated by their 

 focal lengths, thus we have a 3-in., 2-in., 1-in., f-in., |-in., ^-in., 

 i-in., fin., i-in., ^^vci., j-^-in., ^Vi^^-j A-i^^-^ tu-"^^-^ ^^ut these 

 designations do not represent the actual focal lengths of compound 

 objectives, which are far shorter, but they represent the focal 

 lengths of single lenses of equivalent magnifying power. The 

 angular aperture of an object-glass is that angle formed by the 

 extreme rays of light entering the glass from the focus. Large 

 angled object-glasses are dearer than low angled ones. The former 

 show surface markings best, owing to the greater number of 

 oblique rays which pass through them, whilst glasses with smaller 

 angular aperture show various planes of the object to be at the 

 same time in focus. Object-glasses of moderate angular aperture 

 are preferable, unless the microscopist be engaged in special studies 

 requiring a large angled glass, but such may be provided with 

 stops to reduce the angle in order to increase the penetration. 

 Wide angled glasses give more light than narrow angled ones, 

 but the latter have a greater working distance. 



Low powers are all glasses from the 5-in to the f-in. ; medium 

 powers from the |-in., to the -|-in. ; and all object-glasses of shorter 

 focus are called high powers. 



The simplest object-glasses are single bi-convex (PI. IIL fig. 1) or 

 plano-convex lenses ; if achromatic, they consist of a bi-convex and a 

 plano-concave lens (fig. 3). In low compound objectives there are 



