ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
691 
According to experiments made by the author, the apparatus may be 
used as a finder, even with high magnifications, although in these cases 
it is on many grounds preferable to mark the desired point with a 
medium system, and having found it again by means of this, to replace 
the latter by a higher system. 
Value of Artificial Sources of Light.* — Prof. Rogers has recently 
made a series of new experiments on the value of different sources of light 
as regards white light and illuminating effect ; the result is in favour of 
the light of burning magnesium. 
In the experiments the spectra were produced and the intensities of 
the individual colours of the different sources were compared by mea- 
surement with one another. The results were represented by curves, 
aud it was seen that the curve generally corresponded with that of day- 
light with a more or less cloudy sky. With a cloudless sky, on the 
other hand, daylight had a considerably higher effect on the more 
refractive parts of the spectrum in the green, blue, and violet. The 
electric arc-light, according to Nichols, is in these parts weaker than 
the sun or magnesium light, but increases in intensity very strongly in 
the violet and ultra-violet parts. Gaslight naturally showed itself 
brightest in the less refractive parts. 
The temperature of burning magnesium is much lower than one 
would expect, viz. about 1340°, or between the temperature of the so-called 
luminous flame and that of the Bunsen flame. The radiation effect is 
greater with magnesium than with any other artificial light, being forty 
times greater than that of gaslight. Only the electric discharge in 
vacuo is superior to it in this respect. 
Fig. 98. 
Macer’s Reversible Compressorium. — Mr. R. Macer has devised a 
reversible compressorium which can be worked with a high-angle con- 
denser and high-power objectives (fig. 98). It has a vertical compression, 
and the smallest object can be fixed and examined on both sides. It is 
simple in its use and construction, and is made as follows: — In the 
* Central-Ztg. f. Optik u. Mechamk, xii. (1893) p. 143. 
3 B 
1893. 
