C. GENERAL PHYSICS. 189 



TESTING MICROSCOPE OBJECT-GLASSES. 



Dr. Pigott advocates the method of testing the object- 

 glasses of microscopes by examination of the miniatures re- 

 flected from small globules, especially the examination of 

 the sun's image as seen reflected in small globules of mer- 

 cury. In this method, an object-glass of fine quality is 

 screwed into the sub-stage of the microscope in an inverted 

 position. On black velvet there are scattered, from a syringe 

 containing mercury, a number of mercurial globules ; then, 

 by means of a prism, a brilliant light is thrown vertical- 

 ly downward upon them. The object-glass to be tested 

 is now screwed to the nose of the microscope. The two 

 objectives are brought to a central position, so that their 

 axes coincide, and the instrument is then adjusted to form 

 miniatures of the globules for examination. The most beau- 

 tiful effects are seen under sunlight. The miniatures de- 

 velop appearances of marvelous beauty and variety. The 

 aperture of the miniature -making objective should be at 

 least as wide as the objective to be tested, and the lens of 

 the finest quality obtainable. Among the innumerable il- 

 luminated objects that may be used, Dr. Pigott strongly 

 recommends what he calls the fundamental experiment; 

 that is, a disk of intense light as small as possible, viewed 

 from a distance sufficiently great to develop the test dif- 

 fraction rings. It is well known that the surface of the il- 

 luminated globules of mercury becomes more nearly spher- 

 ical as they diminish in weight. The law of the curvature 

 of these surfaces dependent upon the specific attraction of 

 mercury has been investigated by Professor Bashforth, 

 though not yet published. Under direct illumination, a 

 minute spectrum of the sun may be described. The sym- 

 metry, beauty, and fineness of refraction rings exhibited by 

 these miniatures from illuminated globules of mercury are 

 severe tests of the objective, and afford delicate means of 

 adjusting its corrections. Monthly Microscopic Journal^ 

 1875, 147. 



ACCIDENTAL OR SUBJECTIVE COLORS. 



Mr. Plateau states that observations made by himself 

 upon a number of persons of his own acquaintance have 



