structure of Podura Scales. 7 



consist apparently of "illusory beads," and tlie belief now appears 

 to be, with the most unbiassed observers, that they are such. 



The Degeeria domestica, or speckled Podura, when shown 

 opaquely * under a x? th or upwards, is a specially beautiful object. 

 The scales are apparently much thicker than in other species, and 

 the ribbings or ! ! ! markings are of a reddish-brown colour — not 

 beaded, but shghtly constricted at regular intervals, like the short 

 antennae of some insects, and in the deep intercostal spaces there 

 are numerous thin sept^e, or transverse bars, very fine and distinct, 

 of a greyish tint. Both these and the slightly " varicose " spaces 

 on the ribs may be displayed in the form of beads, by dodging the 

 illumination. Where practicable, some form of opaque illumination 

 should always be employed for verifying the structure of these 

 objects, for we are in this case quite free from the errors of 

 diffraction, which more or less accompany objects seen by trans- 

 mitted hght, and cause an indistinctness of outline. 



* The only plan at present known to me of illuminating these and other dry 

 test-objects is the one that I recently described, and consists in mounting them, 

 not on the cover, but on the slide itself. When the light is totally reflected from 

 the upper surface of this at the part where the object is in contact, total re- 

 flexion will not occur, but the light will pass into the object, and in many cases — 

 as with some diatoms and the Podura — illuminate it with sufScieut brilliancy ou a 

 black field to be plainly seen with the highest powers and eye-pieces. 



There are several methods of securing this total reflexion, some of which are 

 described in my paper " On a Method of Illuminating Opaque Objects under the 

 Highest Powers of the Microscope," read before the Microscopical Society on 

 March 26th, 1856, and published in the ' Transactions.' One of them consists of 

 a solid glass parabola with a flat top, ujjon which the slides were placed, with an 

 intervening film of highly refractive medium ; this was made at the time, and ever 

 since has been in use, mostly for viewing aquatic animalcules and living diatoms, 

 &c., for which it is especially convenient. The parabola is first lowered so as to 

 bring its top below the level of the brass table, and the object in a drop of water is 

 covered with thin glass. Tiie parabola is then screwed up till the object is secured. 

 The rotifers are beautifully shown this way; but I have found that for objects on 

 slides its use is not so convenient as the small truncated or nearly hemispherical 

 lens and the ordinary parabola, as this arrangement affords greater facility for 

 traversing the object, as there is suificient play for the lens for this purjjose in the 

 hollow top. 



As some have complained of a difficulty in using this, I repeat the directions. 

 The little lens is first patched on imder the slide with a minute drop of oil of 

 cloves or turpentine where the Podura scales or other objects lay thickest. The 

 slide is then put on the stage with the lens in the top of the parabola. The next 

 thing is particularly to obtain jmrallel light. A large bull's-eye condenser should 

 be employed with its flat side towards the lamp : hold a slieet of paper over the 

 plane mirror, and see that the light falls in the centre, and is of the same area a3 

 the bull's-eye or about fills tlie mirror ; then go through the ordinary adjustments, 

 and if the field is not quite black raise the stop of the parabola : look at the objects 

 with a low power, those only that are biilliantly luminous are on the slide. Select 

 a proper one, and if it does not appear nearly in the centre of under lens, shift 

 this beneath it. One cause of failure may be the absence of objects detached from 

 tlie cover on the slide ; but in English mounted objects, either of diatoms or scales, 

 I have in most cases found a few straggling specimens that have left the cover. 

 Possibly the figure of some of the paraboloids may have degenerated since I sup- 

 plied the original steel template near twenty years ago, which must have been 

 worn out before now ; but this I will see to. 



