USB OF ACHROMATIC CONDENSER. 173 



a distinct image of the lamp. In every case, indeed, in which 

 it is desired to ascertain the eflect of variety in the method of 

 illumination, the effects of alterations in the distance of the con- 

 denser from the object should be tried ; as it will often happen 

 that delicate markings become visible when the condenser is a 

 little out of focus, which cannot be distinguished when it is pre- 

 cisely in focus. . The diaphragm-plate with which all the best 

 forms of Achromatic Condenser are now furnished, enables the 

 observer not only to vary the angle of his illuminating pencils 

 through a range of from 20° to 80°, but also to stop off the 

 central portion of the pencil, so as to allow only its most oblique 

 rays to pass ; and the contrast presented by the aspect of many 

 objects, according as the size and form of the aperture in the 

 diaphragm-plate limits the rays transmitted by the condenser to 

 those of the central or those of the peripheral portion of the 

 pencil, is often so marked, as to show beyond question the great 

 importance of this mode of varying the illumination. When 

 the condenser is employed, the plane Mirror may often be sub- 

 stituted with advantage for the concave ; the chief effect of this 

 exchange being to diminish the quantity of light, without alter- 

 ing the angle of the illuminating pencil. It must be borne in 

 mind, in making such an alteration, that the plane mirror reflects 

 parallel or (if from a lamp) diverging rays, instead of the con- 

 verging rays reflected by the concave mirror ; so that the focus 

 of the condenser is likely to require readjustment. For objects 

 of great delicacy and transparency, the "white-cloud" illumina- 

 tion (§ 58) may be had recourse to with advantage ; or, if it be 

 desired that the illuminating pencil should be free from the error 

 imparted by the double reflection of the mirror, the mirror may 

 be turned aside, and in its stead the lamp (if the observation be 

 made by artificial light) may be placed in the axis of the micro- 

 scope; or the mirror may be replaced by "Dujardin's prism" 

 (§ 57), which will be equally available either by daylight or by 

 lamp-light. 



89. Should it be desired, however, to try the effect of very 

 oblique light upon an object, the Achromatic Condenser must 

 be removed (unless, as in Mr. Sollitt's arrangement, § 130, it be 

 so constructed as to be capable of inclination to the axis of the 

 Microscope), and other means must be employed. The simplest 

 method, where the mirror is mounted on an "arm" (Fig. 29), is 

 to turn it to one side so as to reflect the rays at a considerable 

 angle ; and where this cannot be done, nearly the same effect is 

 produced by placing the lamp in the direction from which it is 

 desired that the oblique rays should proceed, and interposing an 

 ordinary condensing lens between it and the object. Or, if the 

 Microscopist be provided with the means of mounting a " Du- 

 jardin's prism" on a separate stand, he may place it in such a 

 position as to reflect light from any point required : and he may 

 concentrate that light by an ordinary condenser. The possession 



