SCIENCE-GOSSIP. 



57 



NOTE ON MELICERTA R I N G E N S. 

 By W. II. Dallinger, LL.D., F.R.S., F.R.M.S., etc. 



HPHIS small and beautiful denizen of our ponds 

 has become so familiar an object to the 

 amateur, and has so long commanded the interest 

 and close observation of the student, that it may 

 fairly be supposed nothing short of prolonged and 

 systematic study could tell us more concerning 

 it, than from so many 

 contributors is now 

 known. Apart from 

 the beautiful results 

 obtained by the insight 

 and patient researches 

 of Gosse ( x ) confirmed 

 and enriched by scores 

 of subsequent observers, 

 it may be doubted if 

 anything more thorough 

 could be done than that 

 by Mr. Charles Cubitt ( 2 ) 

 and Judge Bedwell (■'), 

 on the characteristic 

 and really wonderful 

 features of this rotifer, 

 all of which has been 

 carefully considered and 

 analysed by the chief 

 authority on this subject, 

 Dr. C. T. Hudson ( 4 ). 



Nor is there any claim 

 in this note for the 

 addition of any fact to 

 our previous knowledge, 

 it is simply a demon- 

 stration of the manner 

 in which a detail fami- 

 liarly known to be 

 constantly carried out, 

 is accomplished. 



No greater perplexity 

 presented itself to the 

 earlier microscopists in 

 the study of this and 

 similar forms, than the 

 extreme limits of space 

 involved in the use of 

 objectives of consider- 

 able magnifying power. 



These frequently rendered certain actions of the 

 organism impossible, or only to be studied when a 

 happy coincidence gave the patient enquirer the 

 fortunate opportunity he could never arrange for 

 himself. Or else, as not infrequently happened, 

 led a vigorous animal under constrained conditions 



May, 18.95.— No. 15, Vol. II. 



Fig. 1. — Melicerta 

 (Drawn from nature 



to perform partially or imperfectly a distinguishing 



habit. 



We are relieved largely in this matter by the 

 large advantages of the apochromatic system of 

 objectives and their accompanying compensation 

 eye-pieces, a system which, as is now well known, 

 provides such perfect 

 chromatic and spherical 

 correction that the aerial 

 image may with con- 

 stantly improved reveal- 

 ing power, be magnified 

 to a limit never before 

 attainable by the eye- 

 piece. The result is 

 that we can obtain 

 increasing power while 

 we leave the focal 

 distance unaffected. 



It is well-known that 

 beyond the axial por- 

 tion of the visual ' ' field ' ' 

 certain colour defects 

 are inevitable ; thev 

 arise from the chro- 

 matic difference of the 

 magnification, and this 

 may be the case al- 

 though the centre of 

 the field may be per- 

 fectly achromatised. 

 The differently coloured 

 images produced by 

 refraction, and which 

 unite to form the one 

 visual image, are differ- 

 ently magnified, that is 

 of different dimensions. 

 The red image is the 

 least, the blue image 

 the greatest. 



The apochromatic 

 system corrects the 

 primary and secondary 

 spectra ; there is, how- 

 ever, still chromatic 

 error remaining, but the 

 1 ' apochromatic ' ' system involves such a construct ii 11 

 of the eye-pieces that they shall possess an equal 

 error of the opposite kind. The image formed by 

 the red rays is greater than that corresponding to 

 the blue rays. In this way, perfect compensation 

 is secured, and the image being unconfused by 



RINGENS, X 100. 



by Dr. Dallinger, 



