686 



NATURE 



\Oct. 24, 1878 



of the mineral is greater or less than that of the balsam. It 

 then follows that / ± ^' is the thickness, as seen through itself, 

 of the amount of balsam of the same real thickness as that of 

 the mineral : the effects of the balsam below and above it, and 

 of the covering-glass, being thus entirely eliminated. If, then, 

 the index of the balsam be m, we can easily calculate that of 

 the mineral (/*) from the following equation : — 



t±d' 



In the case of the hard and brittle balsam used to fasten down 

 the specimen, the value of m is about 1*54 ; but if there be any 

 doubt about the true index, it can be ascertained by special 

 measurements. 



In a similar manner we may determine the index of some 

 unknown mineral by comparing it directly with some other 

 mineral lying near to it, the true index of which is either well 

 known or has been previously ascertained from special measure- 

 ments. For this purpose quartz is often very suitable, since its 

 index varies very little. One great advantage of this method is 

 that specimens may be observed far away from the edge of the 

 section, provided of course that the minerals compared are so 

 close together as to prevent any error due to unequal thickness 

 in different parts. 



It must be borne in mind that, when any mineral has a very 

 powerful double refraction, its apparent thickness, as seen 

 through itself, varies according to the particular ray used for 

 illumination and the direction of the objects chosen to determine 

 the focal distances of the lower surface. There is, however, 

 generally no difficulty in measuring with sufficient accuracy the 

 mean apparent thickness, or that corresponding to some one 

 image, and in calculating out the results accordingly. 



In connection with this subject it may be well to call attention 

 to a somewhat interesting fact. If we have, side by side, two 

 substances of different refractive power, but of the same abso- 

 lute thickness, their apparent thicknesses, as seen through them- 

 selves, vary directly as the velocity with which light moves in 

 them. Indeed, strictly speaking, the determination of minerals 

 in the manner now described depends entirely on an indirect 

 measurement of the velocity with which light is propagated 

 through them in different directions. 



In order to illustrate the practical applications of this method, 

 I will describe the results obtained in the case of a section of 

 dolerite from near Glasgow, which, on an average, is about 

 ^J^th of an inch thick, 



I found that the index of a colourless transparent mineral, 

 filling up cavities between the original minerals, was about i "48 

 or I •49. This exactly corresponds with that of analcime, with 

 which its other optical characters agree. 



Another colourless mineral, also filling cavities, was found to 

 have the indices and other characters of calcite. 



A third colourless mineral, evidently an original constituent, 

 was seen to have a comparatively feeble double refraction, and its 

 index was found to be l'6l. Its general appearance was like that 

 of some felspar, but this index clearly proves that it cannot be 

 any species which contains a considerable amount of alkali, 

 which would greatly reduce the refractive power. The index of 

 labradorite was not previously known, but I find that it is i"6l, 

 and therefore there can be little doubt that the mineral in the 

 section is that species. 



The section also contains a number of transparent reddish- 

 brown crystals, their index of refraction being about 179. This 

 and their other optical characters closely agree with those of the 

 dark augite in the lava of Vesuvius. 



In now concluding this short address I cannot but feel that 

 I have been obliged to omit all allusion to many points of con- 

 siderable practical importance. I have not attempted to describe 

 the subject in such a manner as would enable any one to at once 

 practically apply the method in all sorts of cases. I gave a 

 somewhat full account of one branch of the subject in my 

 address at the meeting of the Mineralogical Society at Plymouth, 

 and entered into the more purely microscopical aspect of the 

 question in my late address at the anniversary meeting of the 

 Royal Microscopical Society. I propose to communicate a 

 detailed paper to the Royal Society as soon as a correct expla- 

 nation can be discovered of certain small but remarkable discre- 

 pancies between mathematical theory and observation. My 

 chief object now is simply to point out what valuable facts may 

 be learned respecting the nature of any mineral by looking 

 through it with a microscope at a ckcular hole or rectangular 



grating. This is a totally different thing to magnifying the 

 mineral itself, or to looking through the mineral at" any dtstant 

 object without a microscope. The success of the method 

 depends entirely upon the optical conditions characteristic of a 

 compound microscope. I have lately greatly improved the 

 apparatus hitherto employed, but the examples already given 

 will, I trust, serve to prove that, even with the less perfect 

 appliances, it was possible to identify in a very satisfactory 

 manner, many of the minerals met with in their microscopical 

 sections of rocks, and thus to determine their constitution with 

 far more certainty than heretofore. 



RECENT OBSERVATIONS UPON THE P LA- 

 CENT ATI ON OF THE SLOTHS 

 jyr JOLY has recently brought before the Academy of 

 • Sciences of Paris the results of a careful examination of 

 the structure of the placenta of the Ai, or Three-toed Sloth 

 (Bradypus tridactylus, Linn.), and proposes important changes 

 in classification, after comparison of this structure with that of 

 certain allied mammals.^ 



It is now six years since we contributed to Nature a notice 

 of the observations of M. Alphonse Milne-Edwards upon the 

 foetal envelopes of another member of the Edentata, the 

 "Tamandua" Ant-eater, published by him in iht A nnales des 

 Sciences Naturelles. This Edentate is there stated to have a 

 " placenta discoidal envahissant." 



The sloths are literally, as Buffon described them, ruminant' 

 animals, in that they have four stomachs, but they are, at the 

 same time, wanting in all the other characters which pertain to 

 Ruminants proper. Linnaeus, on the contrary, classed them, at 

 first among the Primates, but afterwards among the Bruta — the 

 "Edentes"of Cuvier — and his example was followed by Do 

 Blainville. Cuvier placed the " Tardigrades " {Bradypus) at 

 the head of the Edentata, although they possess well-developed 

 canine and molar teeth. 



It will be seen, then, that as regards the position of these 

 animals the embarrassment of the taxonomist has been extreme, 

 as the genus Bradypus has been bandied about from the Rumi- 

 nants to the Primates, and from these latter to the Edentata, 

 Latterly, however, great importance — and with good reason — 

 has been attributed to the structure of the placenta, as affording 

 characters distinctive of the various groups of mammals, and as 

 giving valuable indications of their zoological affinities. The 

 classification of the placenta by Carl Vogt, though scarce a 

 quarter of a century old, into zonary, diffuse, and discoidal, is 

 nowadays acknowledged to be incomplete, nay, even faulty in 

 some of its applications ; for we know now, thanks to the 

 work of Alphonse Milne-Edwards, that if the majority of Ru- 

 minants have a multicotyledonary placenta, the camel, the chev- 

 rotain, and the Tragulus have, on the other hand, one of the 

 diffuse variety. It is the same with the digitigrade Pachyderms 

 (wild boar, &c.), while the plantigrade ones (Proboscidea, 

 Hyracoidea) differ from the first in having a zonary placenta 

 like that of the Carnivora and Amphibia (seals, &c.). In 

 fine, although stated to be so, this organ is neither diffuse 

 nor subdivided among any of the Edentata studied from the 

 point of view of their placentation. Nay, more ; among these 

 animals the placenta offers, according to genera, and even ac- 

 cording to species, differences so well marked that it is neces- 

 sary, following the apposite remark of M. Alphonse Milne- 

 Edwards, to give up seeing between the different types of 

 Edentata affinities as narrow as those which are supposed, even 

 now generally, to exist among them. Carus has represented the 

 placenta of the Ai, or three-toed sloth, as being multi-lobed, 

 but he does not give any precise information as to the number 

 of these lobes, their structure, the extent which they occupy 

 relatively to the membranes of the ovum, and their connection 

 with the uterine mucous membrane [decidua ?], &c. 



The placenta of the Ai examined by M, Joly presented itself 

 under the form of a veritable membranous pouch constituted by 

 the amnion and the chorion, and garnished, on almost all its 

 external surface, with a large number (more than a hundred) of 

 lobes or cotyledons of more or less irregular shape and of very 

 va.riable size, from one millimetre to one or two centimetres. 

 Viewed from the external face of the placenta, these cotyledons 

 appear, some rounded and flat, like Nummulites, others of the 

 form and size of seeds of millet. Others, lastly, much larger^ 

 grouped in numbers together, recall by their aspect the multi- 



• Compies Rendus, August 19, 1878. 



