134 SUMMAKY OF CURRENT RESEARCHES RELATING TO 



Over other yellow or green fluids the copper-chrominm filter pos- 

 sesses the advantage of only letting through a very small part of the 

 spectrum ; if concentrated, only yellow-green rays from A 580— X 560 ; 

 if diluted, A 690— X 545 ; with great dilution orange rays appear ; for 

 these the erythrosin plate possesses very slight sensitiveness. 



A filter roughly resembling the foregoing may be made by adding 

 ammonia in excess to a mixture of copper salts and chromate of potash. 

 This, however, only lets through such green rays as the erythrosin 

 plate is but little sensitive to. The maximum and minimum of sensi- 

 tiveness in the plate lie close together. 



It was found that by using the copper-chromium filter combined 

 with mineral-oil light and long exposure the sharpness left nothing to> 

 be desired with ordinary objectives up to a magnification of 400. After 

 this difficulties arise which are only overcome by the use of apochro- 

 matics, a condenser, and the light-filter. 



(5) Microscopical Optics and Manipulation.. 



Optical Effect of Focusing up or down too much in the Microscope.* 

 —Mr. W. M. Maskell writes that, if when observing Goniiim, the objec- 

 tive be lowered a very little, so as to throw the alga out of focus, and to 

 see, as it were, beyond its surface, not only do the outlines become 

 blurred and indistinct, but a somewhat curious change of colour is notice- 

 able. The whole plant assumes a green ground colour, the spaces 

 formerly visible between the cells being obliterated, and at the samo 

 time an elegant geometrical pattern is produced, with various tints. 

 Four crimson specks appear at about the middles of the four inner cells, 

 and with these as centres four delicate circles of bright yellow interlace 

 each other, the radius of each circle being the distance between two 

 orimson spots. The spots are also connected by narrow bands of lighter 

 red colour. The outer ring of cells appears as composed of pyriform 

 bodies, the points inwards and overlapping, producing thus the semblance 

 of green spokes in the four circles. In each of these cells, on the cir- 

 cumference of the circles, is a crimson spot formed of concentric curves 

 open towards the middle of the plant. By focusing downwards a little 

 more or less the crimson spots or the golden circles may be made more 

 or less conspicuous on the green ground. 



If, again, the object-glass be screwed up, past the true focus, an 

 entirely different effect is produced. Instead of the whole plant appear- 

 ing solid, the spaces between the cells are amplified, and the whole 

 colony seems larger and more scattered ; and the cells, quite disconnected, 

 are now not green, but yellowish-brown, with a broad darker band 

 encircling each. These effects of colour are noticeable not only with 

 a 1/4 in. objective, but also with the 1/8 in., and they may even be 

 made out with the 1 in., though, of course, not well, as the plant then 

 appears so small. 



The author adds, " Of course, I presume that the effects here spoken 

 of are easily explicable : the passage of the light through the semi- 

 transparent green cells, the translucent envelopes, and the empty spaces, 

 producing complementary colours. And in itself the thing is doubtless 

 not of any importance. Yet indirectly it may possess some value, as in 

 p, certain kind of way a warning. From the measurements which I have 



* Sci.-Gossip, 1888, pp. 248-9 (3 figs.). 



