236 PROGRESS OF MICROSCOPICAL SCIENCE. 



separated, is not of itself sufficient evidence. In the cat the whole 

 thickness of the mucosa in the placental zone peels oflf along with the 

 placenta when that organ is artificially separated ; whilst in normal 

 parturition the deeper part of the connective tissue of the mucosa, 

 with the remains of the blood-vessels and tubular glands, persists as 

 a covering for the muscular coat, and forms a non-deciduous serotina. 

 It may be that in Hyrax, as in the cat, only the superficial part of 

 this membrane is shed with the placenta, whilst the rest remains on 

 the zone of the uterus; but this can only be determined by the 

 examination of a uterus immediately after parturition." 



The Rotifer within the Volvox. — The following observations are 

 taken from a new American periodical devoted to microscopical 

 science. The writer says that having watched the above phenomenon 

 on one or two occasions, " and being desirous of keeping a few volvoces 

 for future examination, and having formerly had very poor luck with 

 such attempts, I took especial pains this time. I scalded two bottles 

 thoroughly, and partially filled them with boiled water. When quite 

 cold I placed a dozen volvoces in one bottle, taking them up with as 

 little of the original fluid as possible. After an hour or two I trans- 

 ferred them, one by one, from the first bottle to the second, where 

 they are now probably alone, and are thus far doing well. I examined 

 each one carefully when first caught, and what was my astonishment 

 to find several with rotifers in ■ their interior, the rotifers being ap- 

 parently very busy making a meal. That the inhabitants of these 

 volvoces were rotifers, there can be no doubt. Carpenter describes 

 the presence of amoebfe in the volvox, and explains it by stating that 

 the endochrome-mass of one of the ordinary cells has assumed this 

 condition. The phenomena which I have just seen certainly can- 

 not be thus explained, and I feel puzzled to know how the rotifer got 

 inside the volvox, while the latter ajjpears to be unbroken and con- 

 tinues to swim about in the usual manner, carrying the strange 

 ' entozoon ' with it. That the rotifer is inside is easily shown by 

 focussing down through the volvox. First we see the upper surface 

 of the volvox, then the rotifer, and lastly the under surface of the 

 globe." 



How to Measure the Angular Aperture of Object-glasses. — There are 

 many who possess the microscope with most of its accessories to 

 whom the above question would prove a very formidable one in- 

 deed. We think therefore that the following condensed account, which 

 has been given by Mr. Ingpen in the ' Journal of the Quekett Club' 

 (January), which very briefly and clearly defines the various methods 

 employed will not be without interest : — Down to the year 1854 the 

 method of measuring angular apertiu'es devised by Mr. Lister seems 

 to have been the only one employed. This is described in the ' Phil. 

 Trans.,' vol. cxxi., p. 191, and will be found in ' Quekett on the 

 Microscope,' ed. 1855, p, 497. The microscope, with its objective and 

 eye-piece as in ordinary use, is placed horizontally, a candle is set on 

 a level with it, a few yards distant; the microscope is then turned, 

 till, on looking through the eye-piece, the field of view is bisected, 



