HARDWICKE'S SCIENCE-GOSSIP. 



185 



MICROSCOPY. 



VOLVOX STELLATUS. — In a pond in Epping Forest, 

 on 20tli of M ay last, I was fortunate in finding the 

 Volvox stellatus of Ehrenberg. I believe this rare 

 form of volvox has not been found in England for the 

 past twenty years at least, but if it has I should be 

 glad to know from any one who has found it, when 

 and where it was, as I cannot find any description of 

 it more recent than in Pritchard, other articles being 

 evidently borrowed. My own observations, made 

 within an hour after taking them, lead me to differ 

 somewhat from the conclusion there put forth. It was 

 evidently a very favourable opportunity, for while ex- 

 amining one with x 150 it burst, giving forth a multi- 



% 



(^ 



^ 



Fig. 133. — I, Volvox globator, containing V. stcllaUis ; 

 2, V. stellatus = ^.-'y.^ in. 



tude of microgonidia, thus showing that it was filled 

 with them and that they were not as in V. globator only 

 on the periphery. The stellatus forms occupy the 

 same places in the globator as the ordinary gemma, 

 but do not grow to the size of the young globator. 

 It will be understood that the sphere is covered with 

 the conical protuberances, and that it has no internal 

 marginal sphere, some of the gonidia being inside the 

 cones. They do not appear to be provided with 

 cilia, as I could see none, and they had not the usual 

 rolling motion, but merely floated out of the mother 

 cell as it broke up and soon afterwards discharged 

 their contents. I am led to believe that the stella- 

 tus is a form taken by the volvox in an alternation of 

 generations ; that is, having passed through its other 

 various forms of increase, then at certain seasons 



which are conducive it acquires this stellatus form 

 which being filled with gonidia (?) would thus help to 

 increase them to a wonderful extent, — jfames D, 

 Hardy, Clapton. 



The sign x .—Why does T. J. R. maintain that 

 this sign means more than amplification ? I cannot 

 call to mind any instance in which it is used in any 

 other or additional sense. Let us examine his own 

 example, he says that a tj with an eyepiece, X 500 

 diam. he examines an object with it, and finds many 

 markings brought out. This he considers represents 

 the object perfectly as X500 diam., but supposing that 

 a j'g is constructed which shows many more details, 

 would he be correct in saying that the former glass 

 no longer X 500 diam. or that the new y'g X more than 

 500 diam. ? With both glasses the "ooi of an inch on a 

 micrometer appear of equal length (/g of an inch). If 

 T. J. R. is correct, then the microscopes of 1782 had 

 no magnifying power at all. Resolving power is 

 not dependent on amplification (within certain limits), 

 but first on the angle of aperture, second the obliquity 

 of the illuminating beam. Suppose we try a specimen 

 ol Plcurosigiiiaformosttiii using the y'5 and direct light, 

 the dots are brought out admirably, therefore it is X 

 500 diameters, for the Pleurosigma we substitute 

 Navkiila rhoinbohies, we see the margins and the 

 median line sharply defined, but no trace of lines 

 crossing the valve ; we throw an oblique ray upon 

 it and we find that the object has lines crossing it 

 longitudinally and transversely. We alter the 

 position of the valve, and the supposed lines are 

 resolved into dots. Is the magnifying the same 

 in every case, or does the 500 diam. for Pleuro- 

 sigma become less when rhomboides is examined 

 under the same conditions and again becomes 

 500 when oblique light is used ? Magnifying 

 power is not a variable quantity ; a one inch must 

 always magnify 10 diameters, and if we combine an 

 ocular with its X power, is 10 X by the power of 

 the ocular at any given distance from it, whether jij 

 is X by 5 or 75 by 50, the result as far as magnifi- 

 cation goes is precisely the same. The "ooi of an inch 

 was the apparent length of^'j. I am of course taking 

 10 inches as the standard distance of perfect vision. 

 — Fred. Kit ton. 



Cutting Coal Sections. — Please correct error in 

 my remarks on this subject, for 2nd method read " in- 

 cineration " instead of " maceration." Professor 

 Reinsche's sections of coal arc opaque with the ex- 

 ception of the organic remains contained in them, 

 which are of the colour of amber. — F. K. 



Sections of Coal. — It seems funny that your 

 correspondent should talk of easily making sections 

 of coal which evidently are not real coal at all. Any 

 one with the least knowledge of chemistry, can at 

 once say that the plans given for softening coal 

 previous to making [sections, could not succeed ; but 



