398 



KNo\VLi:i)ni:, 



October, 1912. 



to obtiiiii successful results, the coiideuser and source of li^lil 

 must be carefully centred, so as to properly illuminate the 

 object, and the auxiliary condcnseromployed in front of the lamp 

 must be of such a focus thai the ima),'e which it projects upon 

 the sub-stage condenser tills the entire aperture of the latter, 

 when the principal condition for critical illumination is effected. 

 It then only remains to focus the image of the lamp condenser 

 into the object field by means of the rack and pinion with 

 which the iib-stage is provided. The adjustments of the 

 light having been made so as to suit the re(|uirements of wide 

 aperture oil immersion objectives, attention has to be directed 

 to the objects themselves. In photographing diatoms the use 

 of an apochroniatic objective with a blue light filter is to be 

 recommended, as the shorter wave lengths will act like an 

 increased numerical aperture, and so augment the resolving 

 power. In the case of bacteria, however, the conditions are 

 ijuite different, as we are usually dealing with stained speci- 

 mens, and the filter employed to obtain the necessary contrast 

 must be in keeping with the absorption bands of the dye used 

 in staining. If the attempt be made to increase contrast by 

 reducing the aperture, there is great risk of destroying fine 

 details, as well as the formation of very marked diffraction 

 effects round the images themselves. It is therefore evident 

 that a deal of care is required to obtain the best results. In 

 taking the photograph of bacteria shown in Figure 431 a 2eiss 

 two millimetre homogeneous immersion objective of N.A. 

 1-40 was employed, together with a four projection ocular. 

 The source of light was a paraffin oil lamp, and an exposure 

 of three minutes was given, using an Imperial orthochrome 

 plate ot 200 H. and D. The specimen being stained blue, a 

 yellow screen was used to gain contrast. It may be further 

 mentioned that as the three millimetre apochromat of Zeiss 

 has the same N.A. (1-40) as the two millimetre, the former 

 would, in many cases, be the best to use owing to its greater 

 depth of focus. 



DETERMINATION OF MAGNIFICATION.— Having 

 taken the photograph it is often necessary to know the mag- 

 nification of the image. This is easily found by removing the 

 object and replacing it with a stage micrometer, "another name 

 for a three by one slip upon which is cemented a cover glass 

 ruled with lines separated by intervals of known value." If 

 the distance between the images of two of these lines seen on 

 the focusing screen be measured, the magnification is obtained 

 by dividing the si^e of the image by that of the object. This 

 is shown in Figure 432, which is a photograph of the lines on 

 a stage micrometer one hundredth of a millimetre apart. The 

 distance between the lines is practically nineteen rnillinutrc'S. 

 therefore V'-^inn = l,900 diameters. 



EXPOSURE TABLE FOR OCTOBER.— The calculations 

 are made with the actinograph for plates of speed 200 H. and 

 D., the subject a near one, and the lens aperture F.16. 



Rciiiiirks. — If the subject be a general open landscape, lake half 

 the exposures given here. 



ZOOLOGY. 



By Professor J. Arthur Thomson, M.A. 



T\V(^-TOED HORSES.— Professor K. Skoda, of the 

 \^eterinary College in Vienna, gives a precise account of 

 two horses which had two toes well-developed on the fore-leg. 

 Taking the first case, each fore-leg bore beside the ordinary 



single digit (No. IlL) a second. This showed three joints, 

 but did not reach the ground. There was a metacarpal tor 

 palm-bonel for this second digit, largely fused with the 

 ordinary third metacarpal. Besides this there was the usual 

 tree splint or fourth metacarpal, and there seemed to be actually 

 a rudimentary first metacarpal ! Especially when one looks 

 into the details of the case does it look like a reversion to a 

 remote ancestral type with polydactylous feet. 



EDIBLE SEA SOLTKTS. — It cannot be said that the 

 natives of these islands compare well with others in the 

 experimental study of the edible. We shrink from much 

 that is wholesome and refuse some of our best fishes until 

 they are filleted and re-named. We lag far behind the 

 Japanese, for instance, in gustatory inquisitiveness, for they 

 have learned to make an entree of jellyfish and to make a 

 toothsome delicacy of two species of sea squirt iTethyuin). 

 Mr. A. G. Huntsman points out that very similar forms 

 are readily available on the coasts of Canada, and he 

 encourages experiment by precedent. " The inhabitants of 

 Peru and Chili use as food two species of Pyiira that occur 

 on their coasts, and species of the genus Microcosmiis are 

 exposed for sale in the markets of Southern Europe." There is 

 nothing in a name, of course, but a dish of Microcosiiiiis 

 should meet the wants of a diversity of palates. Most sea- 

 squirts have but little muscle, and that unstriped ; but in the 

 Styelids and Tethyids the musculature is often quite thick. 



NATURAL HISTORY OF SLIPPER-LIMPET.- The 

 .American limpet, or slipper-limpet, known technically as 

 Crcpidiila fornicata, was introduced into England about 

 1880, and has spread rapidly on oyster grounds. Mr. Orton, 

 who has recently studied its natural history, says that there is 

 no doubt that it has been introduced along with .American 

 oysters, on which it fixes itself, and that the introduction is 

 probably going on. There are many interesting features in 

 this addition to the British fauna. It takes special care of its 

 spawn. " It constructs about fifty to sixty membranous bags, 

 into each of which it passes about two hundred and fifty eggs, 

 and as the bags are made and filled with eggs they are closed 

 and fastened together by short cords. These cords are finally 

 all stuck on to the surface on which the slipper-limpet happens 

 to be sitting, so that when by taking away the spawning 

 individual the spawn is uncovered, it looks like a bundle of 

 balloons, each containing a number of eggs." .-Vnother feature 

 is their curious habit of sticking together in long chains, by 

 one individual sitting on the back of another. There may be 

 thirteen in a chain, and the bottom individuals are females, 

 the end individuals males, and those between these are 

 intermediate sex forms between male and female. The fact 

 is, as Orton has shown, that the slipper-limpet is a " protandrous 

 hermaphrodite," i.e., it is first a male and then a female. The 

 eggs develop into free-swimmin.g larvae, which may be scattered 

 far and wide. As the slipper-limpets feed on the same food 

 (diatoms and the like) they compete with oysters; but it remains 

 to be seen whether there is not plenty of food for both parties. 



THE MERMIS FAMILY.— The Nrermithid.ae form an 

 interesting family of threadworms, not unfamiliar to those 

 who are fond of gardening. The mature adults are found in 

 the earth or in fresh water, and so are the first larval st.ages. 

 The second lar\'al forms are parasitic in Tracheata and 

 Gastropods. In the earth the adults are sometimes found 

 rolled up in a spiral or coil, sometimes male and female 

 together. The males arc smaller, thinner, more mobile, and 

 with better developed sense-organs at the head-end. Some- 

 times the eggs are laid just where the female was living ; in 

 other cases, when the ground is very damp, an interesting 

 migration occurs which many observers have noticed. The 

 females creep up plants, e.g., cabbages, probably seeking out a 

 drier place for egg-laying. The young larv.ae leave the ground 

 .and bore actively into beetles, caterpillars, millipedes, slugs, 

 and so on. It is noteworthy th.at no food is taken either by 

 the adults or by the young larvae. All the feeding is done by 

 the second larval forms during the parasitic period. .Another 

 interesting feature is that the Mermithidae difi'er from most 

 other Nematodes in being .able to survive mutilation and in 



