HARDWICKE'S SCIENCE-GOSSIP. 



159 



The wing is similar in most respects to that of 

 Leptogaster, but is broader and stouter, and has the 

 discoidal areolet {d, fig. 93) more symmetrical, like 

 that of a Tabanus, figured in Science-Gossip, July, 

 1875, page 148. The feet are like those of the 

 genus Asilus. When treated with caustic potash, 

 and mounted in glycerine, or glycerine jelly, they 

 make excellent polariscope objects. 



There is another species a little less common than 

 D. rnfipes, which is similar to it in many respects, 

 but may be distinguished by the wings, which are of 

 a most beautiful purple-brown colour. Its name is 

 D. celandica. The other species are much less 

 common ; the only one, therefore, which we shall 

 mention is D.flavipes, a fly which might be con- 

 founded with D. rnfipes, but differs from it in being 

 much smaller, and having a brownish body, and by 

 all its legs being yellow. 



The Asilidce all feed on other insects, from which 

 they suck the juices by means of their powerful 

 mouth apparatus. It is noticeable that in them 

 what are presumably the hearing organs (seated in 

 the halteres, and in the wings near their bases,) are 

 well developed ; but in the antennae, the capsules 

 generally considered to be the smelling organs, are 

 small, and few in number. This, we think, favours 

 the supposition that these senses are so situated : 

 for the sense of hearing would seem likely to be the 

 more useful to them, on account of its being able to 

 warn them of the approach of their prey long before 

 it comes within reach of the sense of smell ; aud 

 afterwards, when the prey is near enough to be 

 seized, it is not smell, but sight, that must guide 

 them in pouncing upon it. The supposition is 

 further favoured by the fact that in the Tabanidce 

 and Muscidm (which, from their different mode of 

 obtaining food, would require the sense of smell 

 rather than that of hearing,) the relative develop- 

 ment of these organs is reversed. 



Of the three genera above described, the Dioctria 

 are the earliest, appearing from May to July, the 

 Leptogaster appears about a month later, and the 

 Asili are found from June to September. The 

 larvae of the Asilidce live in the ground, except 

 those of one of the rare genera, which feed on the 

 decaying branches of trees. 



Frank J. Allen and H. M. J. Underbill. 



MICROSCOPY. 



The Bramhall Oblique Illuminator. — Mr. 

 Kitton, one of our first authorities on all that con- 

 cerns diatoms, has been pleased to express his ap- 

 probation of this invention, and to name it after its 

 inventor. I can confirm all that he has said of its 

 resolving power. Whatever an object-glass is ca- 

 pable of doing, I believe it will enable it to do. In 

 addition to the tests named by Mr. Kitton, I may 



add, that with a Siebert's No. 7 immersion, and the 

 A eyepiece, I have by the aid of the Illuminator 

 resolved the following, which I consider the most 

 difficult tests in their class. Pleurosigma Macrum, 

 Navicula Crassinervis, Frustulia Suxonica, and 

 Amphipleura Pellucida. The only test that has so 

 far beaten me is Stauroneis Spicula, and of that I 

 have "glimpsed" the markings. Before I found 

 out this method of illuminating, I could never 

 really resolve any of the more difficult tests, not 

 even Navicula Rhomboides, in spite of achromatic 

 condenser, spot lens, prisms, and stops of various 

 kinds, but by its aid, that old difficulty N. Rhom- 

 boides, dry or in balsam, is as easy as P. Angulatum 

 was of old. The illuminator is made in two forms. 

 The one, represented in Mr. Kitton' s paper, does 

 for such microscopes as have no sub-stage ; the 

 other, and the best, fits the sub-stage, rising and 

 falling by aid of the rackwork. I use a silvered 

 glass mirror of J-inch in diameter, and a polished 

 metal disc of about an inch, which fit the same 

 holder. On the whole, I prefer the metal ; the 

 chief merits of the invention are its simplicity, 

 efficiency, and cheapness. I use only sunlight or 

 clear daylight, but believe it will work equally well 

 with the lamp. Nothing can be more easy than its 

 use. Throw the light on to the object more or less 

 obliquely, as that particular object requires, and 

 regulate the reflector beneath, as experience alone 

 can determine. I hope my brother microscopists 

 will have as much success in the use of this illumi- 

 nator as I have had, and I am sure they will be 

 satisfied. — John Bramhall, St. John's Vicarage, near 

 Lynn. 



Surirella Gemma. — The resolution of this 

 diatom is not so much a matter of magnification as 

 one of illumination ; whether the oV in- of your 

 correspondent will show the markings depends 

 upon its quality, its correction, and upon the illu- 

 mination used. A good •§■ in., ■+$ in-, rs ln -> or 

 T Vin. will show these markings beautifully, but it is 

 quite possible that the ¥ V in. will not do so, although 

 it ought. I have, for instance, not resolved the 

 lines into heads yet with my ■£$ in., whilst my Boss's 

 yo in. shows them splendidly. I use the narrow 

 side of the flame of a paraffin lamp, place a bull's-eye 

 condenser with its convex side next to it, and 

 obr,ain thus parallel rays on the mirror or on the 

 rectangular prism. I always interpolate a blue 

 light modifier. Very oblique rays being essential 

 for the resolution of Surirella gemma, a large dark 

 ground spot or rectangular stop of the condenser 

 must be employed, and the latter, of course, must 

 be racked up rather high. I have, however, ob- 

 tained the best results by the use of one of Wen- 

 ham's paraboloids. I put the darkground stop flush 

 with its apex, and place this about i in. below 

 the object. By changing the position of the mirror 



