550 Progress in Science. [Otober, 
such molecular mobility that it is difficult to obtain an ingot of homogeneous 
composition. The melting-points of a series of alloys of silver and copper 
are represented graphically, and the curve exhibits a somewhat rapid fall from 
pure silver to the alloy employed for our silver coinage, which contains 925 
parts of silver in rooo of the alloy, and corresponds approximately to the for- 
mula Ag,Cu. The alloy of lowest melting-point is represented by the simple 
expression AgCu. Liquidation appears to result from unequal cooling of the 
molten mass, and is modified if the cooling be retarded. The experiments 
made on cubes of various alloys showed that the molecular alrangement is 
largely dependent on the rate of cooling, 
Under the name of Dysiot a new alloy has been introduced in Germany. Its 
analysis has yielded, copper, 62°30; lead, 17°75; tin, 10°42; zinc, 9:2; iron, 
traces. 
According to Mr. Sergius Kern the chrome-iron ore of the Ural mountains 
has been used in the preparation of a chrome-iron alloy, which possesses a re- 
markable degree of hardness, and may be used as a substitute for spiegeleisen 
in the preparation of steel by the Siemens process. 
Microscopy.—Mr. H. J. Slack* supplies the following valuable information 
respecting the use of Mr. Wenham’s “ Reflex Illuminator.”+ “If Mr. Wen- 
ham’s Reflex Illuminator is used under the circumstances for which he especi- 
ally contrived it little difficulty will be found with suitable objects. The light, 
as he explained, penetrates only where the object makes a new surface on the 
slide, and ‘ adts,’ to use one of his familiar phrases, ‘like a hole in a dark lan- 
tern.’”” The effect is so admirable upon many objects, such as scales of insects, 
certain micro-fungi, minute alge, desmids, diatoms, &c., that every one who 
has successfully tried it must wish to add to its range of utility, and this may 
be easily done. It will be found that most balsamed objects, and many in 
which the covering glass lies very close to the slide, give with it so much false 
light when ordinary objectives are employed that the result is very unsatisfac- 
tory. This false light will be found in many cases so oblique that it can be 
got rid of by using an objective with a small angle, or temporarily reducing the 
angle of an ordinary high power by a movable stop. For example, a slide of 
Surirella gemma and this illuminator exhibited no false light with a glass of 
about 7°; some, but not much, with a fine one-fourth made on Mr. Wenham’s 
new formula, and having an angle of 150°; too much to be endurable with 
Powell and Lealand’s immersion one-eighth full aperture ; and none with the 
Same glass and with a stop limiting the rays admitted to about go’. Many 
slides of butterfly and other scales taken at random from a cabinet become 
manageable with reduced apertures, and the effeéts, when the plan succeeds, 
are very curious, beautiful, and instru@ive. Mr. Wenham has alluded to the 
changed aspects obtained by rotating the apparatus when employed upon the 
so-called Podura scale Lepidocyrtus curvicollis, and similar observations may 
be made with regard to Lepisma scales, and those of various inseés allied to 
Podura. Indeed it is not prudent to pronounce an opinion upon any scale of 
difficulty until this method has been tried and all the aspects it produces con- 
sidered in their mutual relations. It is by no means intended to advise micro- 
scopists against the use of this apparatus with large-angled glasses upon 
objects mounted so as to be fit for it; but when slides fail the observer is 
recommended not to abandon the plan, but to reduce the angle of the glass 
and try again, and with good chances of success. The apparatus has a re- 
markable power of increasing both the penetration and resolution of good 
objectives. 
* Paper read before Royal Microscopical Society, 2nd June, 1875. 
1 “ Quarterly Journal of Science,” vol. ii., p. 400. 
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