248 



HARDWICKE'S SCIENCE-GOSSIP. 



the cost of instruments, and so may prove invaluable 

 when the instruments are out of reach. I have for 

 years tried to help friends with information on the 

 subject, but for want of pictures of the clouds my 

 instructions have been misunderstood or forgotten. 



This little book was greatly wanted, and will be 

 warmly welcomed by all who are interested in cloud- 

 lore. 



John Browning. 



THE OPTICAL EFFECT OF "FOCUSSING 

 UP OR DOWN" TOO MUCH IN THE 

 MICROSCOPE. 



By W. M. Maskell, F.R.M.S. 



THE annexed sketches represent the optical 

 effects produced in some cases by bringing the 

 objective of a microscope either a little too near to, 

 or a little too far from, an object to be properly " in 

 focus." The object represented is a freshwater Alga 

 (Gonium pector ale) common in most pools, probably 

 all over the world. The specimen represented was 

 observed in New Zealand. The figure a shows the 

 Alga as it appears under a Beck's J-inch object with 

 " Kelver " eye-piece, in proper focus. Gonium when 

 alive travels somewhat rapidly through the water, but 

 a very small drop of alcohol on the slide will bring it 

 to rest, when its component cells and their flagella 

 can be observed. It then exhibits an arrangement of 

 sixteen more or less orbicular cells in a square colony : 

 their colour is bright green, with a translucent 

 envelope round each one. The diameter of the 

 whole plant and of its cells varies, but may average 

 about ^-inch for the whole plant, and ^'^-inch for a 

 cell. If now the objective be lowered a very little, so 

 as to throw the Alga out of focus and to see, as it 

 were, beyond its surface, as shown in the figure b, not 

 only do the outlines become blurred and indistinct, 

 but a somewhat curious change of colour is noticeable. 

 The whole plant assumes a green ground-colour, the 

 spaces formerly visible between the cells being ob- 

 literated, and at the same 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 crimson 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 circumference of tha circles, is a crimson spot 

 formed of concentric curves open towards the middle 

 of the plant. By focussing 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, as shown in the figure c, an entirely different 

 effect is produced. Instead of the whole plant 

 appearing solid, as in b, 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 

 J-inch objective, but also with the \ : and they may 

 even be made out with the 1 -inch, though, of course, 

 not well, as the plant then appears so small. I have 

 carefully examined a good many (perhaps fifty) 

 specimens of gonium to observe the changes men- 

 tioned, and have found the same in all, provided the 

 plants were not mutilated. In cases where two or 

 three cells were missing the colour-changes were not 

 easily observed. 



Of course, I presume that the effects here spoken 







000^ 



Fig. 91. — Gonium perforate (Mueller) ; a, its true focus ; 

 b, object-glass lowered too much ; c, object-glass raised too- 

 much. 



of are easily explicable : the passage of the light 

 through the semi-transparent green cells, the trans- 

 lucent 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 a certain kind of way a 

 warning. From the measurements which I have been 

 able to make I imagine (my fine adjustment not being 

 graduated there is no attempt at complete accuracy) 

 that the distance through which the ^-inch objective 

 passes from a to b is not more than the T | D of an inch : 

 and from a to c about the same, or rather less. This 

 is accomplished by a very slight turn indeed of the 

 milled head of the fine adjustment. In the case of 

 Gonium perforate it is usually pretty clear when one 

 has the plant properly in focus, especially as the view 

 of the flagella comes as a guide. But there are many 

 objects as to which it may be supposed that so small 

 a difference as -r^-inch may not seem to throw them 

 out of focus whilst in reality they are so to an extent 

 which might cause error. Query : might the striae 



