464 Chronicles of Science. [July, 



11. PHYSICS. 



Light. — The Kev. Father Secchi has made an interesting observa- 

 tion on the Hght emitted from the planet Uranus. This light, it 

 appears, differs from that of the rest of the planets belonging to our 

 solar system. Its spectrum exhibits broad absorption lines, so that 

 the yellow colour is almost entirely absorbed. While it is clear that 

 the light emitted by this planet is reflected light from om- sun, it 

 is evident that the surface of the planet modifies that Hght in the 

 same manner as do coloured bodies. 



Mr. Crookes has recently sent a paper to the Eoyal Society, 

 in which he describes a spectrum-microscope which he has devised 

 in order to obviate the disadvantages of the ordinary instrument. 

 The principal features of the new apparatus are the sub-stage and 

 the box of prisms. The former carries a shding-plate to hold the 

 slit and apertures, a spring stop and screws for adjusting them, and 

 a^ reversed object-glass. The slit and this object-glass are about 

 2 inches apart, and if reflected hght is passed along the axis of the 

 instrument, the object-glass forms a very small image of the slit in 

 front of it. The direct-vision prisms consist of three flint and two 

 crown, fitted in a box screwed into the end of the microscope. By 

 means of a pin they are thrown in or out of action. The object- 

 glass screws on in front of the prism box. By taking the illumina- 

 tion from the sky or a white cloud, Fraiinhofer's lines are visible, 

 and by direct sunhght they are seen in great perfection ; the disper- 

 sion is sufficient to cause the spectrum to cover the whole field, and 

 the achromatism of the lenses being nearly perfect, the lines from b 

 to G are practically in the same focus. When the hght is good, the 

 appearance of the spectrum, and the power of grasping faint lines, 

 are greatly improved by dividing the light with a Wenham prism, 

 and using both eyes ; whilst the stereoscopic effect thereby commu- 

 nicated to some absorption and interference spectra, throws a new 

 light on the phenomena. 



By using a spirit-lamp instead of the illuminating lamp, the 

 instrument answers admirably for examining flame spectra. The 

 characteristic yellow, crimson, or green lines are seen beautifully 

 sharp, on introducing sodium, lithium, or thallium into the flame. 



By means of this spectrum-microscope Mr. Crookes has observed 

 some curious optical phenomena of opals. " 



If an opal which emits a fine broad crimson hght is held in 

 front of the slit of a sj^ectroscope, or spectrum-microscope, at the 

 l)roper angle, and the source of light is moved so as to shine into 

 the spectrum apparatus through the opal, the appearance is observed 

 of a luminous spectrum with a jet black absorption band in the red. 



From these and other experiments it has been found that those 



