250 HOW TO WORK 



become more concentrated, the greatest care will be required to 

 avoid injury to the lenses by the intense heat. 



Dr. Maddox has lately used two large plano-convex lenses super- 

 posed with a large central stop Dr. Woodward's method as described 

 in one of his communications ; also the condenser of two or three 

 plano-convex lenses as recommended by Mr. Wenham, but with 

 moveable stops or diaphragms ; the latter are placed nearer to or 

 farther from the largest lens, the distance being regulated by 

 trial. 



Prof. Rood, of New York, for his higher powers employed a 

 Wollaston doublet, having an angular aperture of 44 as a condenser. 

 He used one of Liebig's silvered mirrors in place of the ordinary 

 amalgam mirror. 



M. Neyt replaces the common solar reflectors by a large prism 

 with parallactic motions ; to condense the rays an achromatic con- 

 densing lens of 2 1 inches diameter is used, and to concentrate them 

 still more, 3 other converging lenses are placed in its focus in such a 

 manner that they can be used together or separated to meet the 

 power of the objective. He likewise has the objective corrected to 

 make the chemical and visual foci agree. In order to render infu- 

 soria stationary while they are photographed, he uses a voltaic stage, 

 so that he can make contact with the poles of a Daniel's battery 

 or induction coil at the proper moment. The shock suddenly kills 

 the little beings and enables him to secure an image, when other 

 wise, from their rapid movements, it would be a mere accident 

 if the animalcule remained in the field of view, or in the desired 

 attitude. 



The Rev. Mr. Reade has proposed a very ingenious method of 

 using his hemispherical condenser with a solar condenser. The rays 

 furnishing light and those giving heat having different degrees of 

 refrangibility, we have here the cone of light-giving rays formed within 

 the cone of the heat-giving rays, the principal focus of the latter being 

 at a greater distance from the lens than the former. When these rays 

 are permitted to cross the axis, their respective situations are reversed. 

 On arranging the hemispherical lens, so that it shall be separated 

 from the principal focus of heat by the sum of its own focal length, 

 the principal focus for light will be found at a greater distance than 

 its own focal length ; hence the heat-giving rays will be rendered 

 parallel, and the light-giving rays will be made to converge to a 

 second focus furnishing light of much intensity separated from the 

 heating rays. Means for using an achromatic object-glass for the 

 solar microscope without endangering its injury are thus supplied. 



Professor Gerlach uses a plano-convex lens with a concave 



