GO 



HARDWICKE'S SCIENCE-GOSSIP. 



poured slowly upon one corner, the plate slightly 

 inclined so that the liquid spreads over its upper 

 surface, and then tilted up so that the surplus collo- 

 dion drains off again into the bottle ; when the 

 collodion has just ceased to drop, the plate is placed 

 upon a dipper aud gently lowered without a pause 

 into the nitrate of silver bath, moved up and down 

 for a few seconds, then left at rest in the bath for 

 about five minutes, when it may be carefully removed 

 and stood on blotting-paper for half a minute to 

 drain. (From the time the plate is coated with 

 collodion till it is cleared all operations except 

 exposure must be carried on in a dark room. A bull's- 

 eye lantern with a piece of yellow glass substituted 

 for the bull's-eye should be used to light the 

 operator.) 



The plate is next placed film downwards in the 

 frame and the dark slide-door fastened ; then the 

 dark slide is carried into the room where the micro- 

 scope has been arranged, slid into the camera, and 

 the shutter drawn up. The length of exposure 

 varies, and can only be ascertained by experiment ; 

 as a guide it may be roughly stated at from four to 

 six minutes for an inch objective with a circle on 

 the focussiug-glass about three inches in diameter : 

 with lower powers under the same conditions the 

 time of course decreases. The time of exposure 

 having elapsed, the dark slide is carried back into 

 the dark room, the plate removed, held horizontally 

 by a plate-holder, and enough of the iron developer 

 to flood it poured quickly on ; the liquid by a slight 

 movement of the plate being kept in constant 

 motion backwards and forwards, the image will 

 appear, and when its detail looks perfect, hold the 

 plate under a tap and allow a steady stream of water 

 falling upon it to wash off all trace of the developer. 

 Now hold the negative up to the light and examine 

 its back with a pocket lens ; if sharp in every point 

 and no stains visible, the next step will be to 

 intensify it. 



Equal parts of the Redevelopers, No. 2 and 

 No. 1, are poured into separate glasses ; the plate 

 flooded by the latter, which is then poured off into 

 the glass containing the former, and the mixed 

 solutions poured back on the plate ; after a few 

 seconds the liquid is poured off again into the glass 

 and the negative examined; if the flame on the 

 lantern can scarcely be seen through the blackest 

 parts, the process must be stopped by washing 

 thoroughly under the tap ; but if this is not the 

 case, the mixed redeveloper must be again poured 

 on ; and this must be repeated till the negative is 

 sufficiently black, care being taken, however, not 

 to repeat this too often, or "fogging" will be the 

 result. 



It now only remains to remove the uudecomposed 

 bromo-iodide of silver from the film ; this is done 

 by pouring the cyanide solution, which can be used 

 again and again, on the plate; when cleared, it is 



! 



again put under the tap, thoroughly washed, set 

 aside to drain, and when perfectly dry gently and 

 evenly warmed, a varnishing solution poured upon 

 it just as described for the collodion ; then when the 

 varnish ceases to drop, it is held before a fire till a 

 bright and hard surface has formed. The negative 

 is now finished and ready to print from. 



NOTES ON THE DIPTERA..— III. 



The Muscid^;. 



nnO but few persons, probably, does the name 

 J- Muscidse convey the idea of anything different 

 from the house-fly, the blue-bottle, and the dung- 

 fly. These three insects are so common as to need 

 no description, but there are a great many 

 species nearly, if not quite, as plentiful, whose 

 existence is commonly ignored. The Muscidse are 

 by far the largest family of the Diptera, and even in 

 the British islands, notwithstanding the smallness of 

 their area, the number of species is countless ; so 

 that many are as yet undescribed and unnamed. 



The Muscidae differ widely from the Bombylidse 

 aud Tabanidse, described in our previous papers, being 

 separated from them by many families. They are 

 closely related to the Conopidoc, the Despunculida 

 aud the (Estridse. -, 



Fig. 28. Bucentes geniculatus, x 7 diam. The antenna, feet, 

 and wing of one side only are represented. 



In consequence of the great size of the family, it 

 is divided into sub-families. These are arranged in 

 two divisions— Calypteratae and Acalypteratse. These 

 terms (derived from KaXvirrt'ip, a covering) refer to 

 the form of the alula (al, fig. 30), which in the Ca- 

 lypteratae is sufficiently large to cover the haltere* 

 while in the Acalypteratae the alula is so small as 

 generally to leave the haltere altogether uncovered. 

 The Calypteratse are usually robust, and are more or 

 less like the blow-fly and house-fly, which are the 

 most familiar examples. The Acalyptevatas are 

 mostly slender, and the dung-fly is a good type of 

 them. 



We will first notice a few of the Calypteratae, and 

 begin with a common little fly, named Bucentes 

 geuiculatus, belonging to the sub-family Tachinides. 



