WITH THE MICROSCOPE. 253 



In the ordinary Wollaston doublet the chromatic aberration is 

 not corrected, but this does not cause any serious difficulty, as by 

 varying its distance, the blue or chemical end of the converging cone 

 of rays can be used to furnish a field of bluish light. Some con- 

 siderable care is needed in the adjustment of the condenser, which- 

 ever kind be employed, so as to equalise the illumination and avoid 

 sun spots when the mirror is used. Mr. Traer got rid of these by 

 making the distance between the object and concave mirror rather 

 more than its focus. The chief aim is to have the full amount of 

 light that will furnish a distinct image on the ground glass. Some 

 make their focal arrangements in the objective, illuminating the 

 object with daylight or a less intense illumination than is to be 

 used in taking the photograph. Dr. Maddox found that in doing this 

 he seldom secured the best focus, therefore he prefers to focus in 

 sunlight condensed upon the object, using an examining eye- piece. 



In using the polarising apparatus for the production of photo- 

 graphic images in some objects which from their great transparency 

 and delicacy are not well rendered in the ordinary way, or some 

 detail lost by using common light, a little care is required. The 

 polarising prism is as usual placed beneath the object, the analyser 

 directly over or behind the objective, and the best appearance 

 sought by the rotation of the lower prism. Mr. Thos. Davis, who 

 has furnished in the Microscopical Journal for the years 1863 

 and 1864, many details connected with the application of photo- 

 graphy to delineating delicate crystals, states that he finds when the 

 object appears best illuminated by the ray which has been reflected 

 from the mirror and transmitted through the polarising prism, the 

 image in the camera was often only partially distinct, and needed a 

 readjustment of the mirror to procure an image that would develope 

 uniformly. He employed a No. i eye-piece, and magnified some 

 of the crystals, as tartar emetic to 50 diameters, tartrate of soda, 

 sulphate of copper and magnesia, and santonine to 40 diameters. 

 Excellent woodcuts from these photographs were given in the 

 Journal to illustrate his observations. To these I must refer the 

 reader. 



333. Artificial Light- Mr. Shadbolt many years ago obtained 

 some beautiful photographs by lamp light. A small camphine or 

 paraffin lamp was placed so that the flame was in the axis of the 

 microscope. A plano-convex lens of about \\ inch diameter with 

 its flat side to the lamp, and a second smaller one of about i inch 

 in diameter and 3 inches focus, were arranged so as to concentrate 

 the rays of light without forming an image of the flame, pi. LIV, 

 fig. 336. The first is placed at such a distance from the lamp as 



