WITH THE MICROSCOPE. 251 



mirror. These are placed at such distances apart that the two foci 

 meet when the convex surface of the plano-convex lens is turned 

 towards the mirror, pi. LIV, fig. 337. 



Dr. Woodward's plan has been already adverted to in p. 237. 



Dr. Moitessier gives the following method : the parallel rays 

 from the solar mirror are received on a bi-convex lens and conveyed 

 to the other extremity of the tube holding the lens, in which, slides 

 by rack and pinion a smaller plano-convex lens. According to the 

 position of the latter, the emergent rays are rendered either parallel, 

 or diverging if placed beyond the principal focus of the large lens. If 

 placed within the luminous cone before being brought to a focus the 

 rays are rendered more convergent, and this forms the general 

 arrangement for high powers. If the small image of the sun thus 

 formed be made to coincide with the surface of the object to be pho- 

 tographed, the phenomena of interference from diffraction are avoided, 

 but this involves an alteration in the respective distances apart of the 

 lenses for different objectives, or the same objective altered in its 

 focus to correspond with any deviation in the distance of the screen. 

 He likewise substitutes for the small condensing lens, a diverging 

 one placed within the focus of the large lens to procure a cone of 

 concentrated parallel rays. These can be again rendered convergent 

 by a small lens. He also receives upon finely ground glass the con- 

 verging rays from a large condenser with a longer focus at some 

 point before coming to a focus. This circle of light then becomes 

 a radiant for the small condensing lens. Thus there is much less 

 diffraction, and although the time of exposure is considerably in- 

 creased, the plan meets the general requirements. The drawings 

 represented in fig. 338, pi. LIV, will illustrate these different 

 methods. 



The mode adopted by the Abbe' Count Castracane is to allow the 

 solar rays to be refracted by a large prism, with a dispersive power 

 capable of giving a wide spectrum before falling on the condensing 

 lens ; a diaphragm being interposed to allow passage only to the rays 

 from the blue end of the spectrum. In this way homogeneous light, 

 in which the actinic power is chiefly situated is obtained, the defects 

 arising from chromatic aberration are avoided, and a more perfect 

 definition results. 



Dr. Maddox found when using the blue cone of rays formed by 

 Abraham's achromatic prism, a great tendency in the object, if 

 very thin and transparent, to be confounded with the field, and the 

 negative to be useless for obtaining positives for the lantern. Care is 

 required not to employ any form of sub-stage condenser of a larger 

 angular aperture than the objective in use. In practice a saving of 



