NOTES AND MEMORANDA. 
205 
The Fine Adjustment of English Microscopes. — Many of the foreign 
writers on the microscope condemn the form of this adjustment, 
because they say that on account of the amplification constantly 
varying in consequence of the variation in the length of the tube, 
precise micrometric measurements are impossible. Dr. Van Heurck, 
the Belgian botanist and microscopist, writing in the ‘ Bulletin ’ on 
the new oil-immersion object-glass and the advantages it offers in 
dispensing with any correction adjustment, says, that it is curious 
that those who have thus criticised the fine adjustment should never 
have criticised the correction collar, for to that their objections 
particularly belong. By very precise experiments made with a cob- 
web micrometer, he satisfied himself that even with a gV difference 
of amplification was insignificant, when with a Boss microscope the 
tube was lengthened or shortened to the full extent of the fine adjust- 
ment. With, however, an or variations in the amplification 
of 100 diameters were obtained from the different positions of the 
correction from 0 dry to the extreme limit for immersion. 
The Ornamental Colours of Daphniadce. — A long and interesting 
paper on this subject by Dr. A. Weismann appears in Siebold and 
Kolliker’s ‘ Zeitschrift,’ vol. xxx. suppl. 123, illustrated with a plate 
in colours. Dr. Weismann, it will be remembered, originated some 
elaborate investigations into the “ terrifying ” colours of animals. 
The author thus summarizes the results of his researches : — 
1. Only a small number of Daphniadas have variegated colours, 
which are for the most part developed in both sexes, seldom only in 
one, and partly in equal and partly in different degrees. 
2. This colouring must be regarded as a decoration which was 
first acquired by one sex alone (probably for the most part by the 
males), but afterwards transferred in most cases to the other sex also. 
It is conceivable that this transfer was materially accelerated by the 
introduction of an alternating sexual selection, so that at the com- 
mencement of every sexual period, the males, at that time few in 
number, chose the most beautiful females, but towards the end of the 
sexual period the females made the selection from among the relatively 
more numerous males. 
3. The acquisition took place probably at a time when already a 
part of the year’s breed multiplied only parthenogenetically. From 
the constant difference in colours between neighbouring colonies it 
may be concluded with some probability that the development of the 
colours only began after the isolation of the colonies, i. e. after the 
Glacial period in Northern Europe. 
4. The transfer took place in a threefold direction, according to 
the law of homochronic transmission of Haeckel, modified by the 
gradual “ retrogression of character,” — first to the other sex ; secondly, 
towards the stage of growth when the sex is undeveloped, or at least 
full growth is not yet attained ; and thirdly, to the parthenogenetically 
produced generations. In all three cases the different species which 
are provided with ornamental colouring are found in different stages, 
the highest stage, i. e. the perfect transfer of the colouring to both 
