Translation of Professor Abbes Paper on tlie Microscope. 191 
cortical layer, and I believe we may regard tlie rod-like corpuscles 
here present as a peculiar modification of tbe trichocysts which in 
many other Infusoria are developed in the cortical layer of the 
body. The projectile tongue-like organ is one of the most remark- 
able features of Didinium ; we must know more, however, than 
Balbiani has told us of it, before we can decide on its real import. 
It is not improbably a pseudopodial extension of the protoplasm. 
Balbiani has followed the Didinium through the process of 
transverse fission. This is preceded by the formation of two new 
wreaths of cilia, between which the constriction and division take 
place, each half previously to actual separation developing within it 
such parts as it had lost in the act of division. The only part 
which in this act becomes divided between the two resulting animals 
is the nucleus. The so-called nucleolus was not seen by Balbiani ; 
and though he observed two individuals in conjugation by their 
opposed oral surfaces, he never witnessed anything like the forma- 
tion of eggs or embryos. 
I believe I have now laid before you the principal additions 
which during the last few years have been made to our knowledge 
of the Infusoria. But though it will be seen that the labourers in 
the special field of microscopical research, to which I have confined 
this address, have been neither few nor deficient in activity, it must 
not be imagined that the subject has been exhausted, or that many 
questions, more especially such as relate to development, do not yet 
await the results of future investigations for their solution. — Anni- 
versary Address to tlie Linnean Society , May 24, 1875. 
III. — Extracts from Mr. H. E. Fripp’s Translation of Professor 
Abbe’s Paper on the Microscope. 
A careful consideration of the means at the disposition of the 
optician, and a critical comparison of the difficulties serving as a 
guide to the discussion of the conditions influencing them, have 
led me to the conclusion that lenses and systems of lenses of 
which each part has prescribed dimensions, can be executed with 
an exactitude that fairly ensures correct action, and with greater 
facility than any other mode of procedure offers for the fulfilment 
of tbe same conditions with equally good results. 
In the workshops of C. Zeiss, of Jena, the construction of 
objectives, from lowest to highest power, is regulated by strict 
calculation for each single part, each curve, each thickness of glass, 
each degree of aperture ; so that all guesswork and “ rule of 
