1871.] Light. 553 



transmitted through it. The alcoholic solution, on the contrary, is of a red 

 colour, and does not exhibit any dichroism. 



The spectrum of lightning has been examined by Mr. J. Gibbons during a 

 severe thunderstorm, but having only a pocket spectroscope, he was unable to 

 determine the position of the bright bands with any degree of accuracy. Two 

 narrow blue-green bands most frequently made their appearance ; they were 

 somewhat narrower than those of the chloride of copper spectrum, and sepa- 

 rated by a wider interval. At times when the flashes were particularly vivid, 

 bands of various degrees of luminous intensity and of breadth were distributed 

 over the whole spectrum ; and, during the occurrence of a flash en zigzag, a 

 group of narrow ones — two being very brilliant — appeared in the more 

 refrangible part of the red, besides those bands in the blue and green, which 

 were always most conspicuous. The complexity of these spectra gave the 

 author the impression that this department of meteorological chemistry would 

 prove a most promising field of investigation. 



Microscopy. — The cells of pure tin now extensively employed in mounting 

 microscopical objects have been the cause of some mischief in the case of 

 tissues injected with chromate of lead and mounted in fluid (probably slightly 

 acid). The lead in the coloured injection has been reduced, and can be seen 

 in numerous crystalline patches, while the yellow tint has almost totally dis- 

 appeared ; the whole combination of cell, fluid, and easily reducible metallic 

 salt, probably formed a miniature battery, and hence the spoiling of the 

 preparation. The use of these cells should always be avoided in mounting 

 tissues injected with vermillion, chromate or carbonate of lead, Prussian blue, 

 or other metallic pigments, glass cells being preferable and incapable of doing 

 injury. However, for the majority of mountings in fluid and all dry prepara- 

 tions the metal cells may be used with perfect safety, no preservative fluid 

 at present used by microscopists acting upon them. The above-mentioned 

 case must be regarded as purely exceptional, and by no means as prohibiting 

 the employment of these economical and useful cells. 



A fortunate accident has enabled Mr. Wenham to establish the reality of the 

 appearances known as the \ markings on the Podura scale. In a slide of 

 Lepidocyrtus curvicollis (the "test Podura") prepared by Mr. S. J. Mclntire, 

 a fracture caused by the slipping of the cover has left some of the ends of the 

 much disputed markings projecting ; owing to the toughness of Podura scales 

 such fragments are difficult to obtain, and opportunities for such observations 

 seldom occur. The Podura difficulty has been solved in precisely the same 

 manner as that of nodular or beaded structure of the Diatomacece, in which 

 the examination of fragments revealed the truth; the varied appearances 

 presented being caused by alterations in the illumination and focus and adjust- 

 ment of the object-glass. The late R. Beck, Mr. Hennah r Dr. Pigott, and 

 others have shown how easily false appearances are to be produced,, especially 

 in transparent bodies, such as glass rods, bosses, &c. 



Mr. N. E. Green has employed the lime-light for the purpose of illuminating 

 microscopical objects viewed with high powers by reflected light. The ob- 

 jectives used have been £th and J 2 th by Ross r and J^th by Gundlach,. of 

 Berlin. The long working distance of the latter objective- has been found 

 especially serviceable. The lime-light was placed about six inches from the 

 microscope, and concentrated upon the uncovered object by means of a small 

 condensing lens. The surfaces of Biddulphia, Triceratium, and Isthmia 

 exhibited an appearance compared by Mr. Green to that of a nutmeg grater,, 

 but in his drawings closely resembling lunar craters. The ** beads " or 

 " hemispheres " of Pleurosigma Hippocampus and P. formosum were distinctly 

 seen, and also those of P. angulatum, but in this instance it required the most 

 careful adjustment of the light to develope their elevations. Mr. Green 

 explains the conflicting appearances exhibited by these Diatomacece when 

 examined by transmitted light by considering that the hemispheres really 

 surmount cylinders of a perceptible length, as seems to be proved by ex- 

 amining the edge of a broken or abraded specimen, or by focussing down- 



