HARDWICKE'S SCIENCE-GOSSIP. 



199 



The angle of aperture of these reflectors was not 

 much inferior to that of second-quality refracting 

 lenses, but their performance was very far below 

 that of the commoner achromatic combinations of 

 the present day. This arose from several causes, 

 the following being the principal : — .First : The 

 figure of the concave metal. This should be a true 

 ellipse, a figure very difficult to attain. Second : 

 the plane mirror must be perfectly flat. This is 

 even of more difficult attainment, and can only be 

 procured by working three metallic surfaces suc- 

 cessively against each other. And supposing these 

 difficulties overcome, there remains the adjustment 

 of the two metals. In addition to these hindrances, 

 the great loss of light arising from so many reflec- 

 tions of course has seriously injured the perform- 

 ance of the reflecting engiscope. 



The reader will, no doubt, be curious to learn the 

 cost of these instruments. According to Mr. 

 Pritchard's catalogue, " the improved American 

 engiscope, as described in ' Micrographia,' " ranged 

 from £18 to £35. 



This catalogue was published in 1837, the re- 

 flecting microscope holding its ground even after 

 achromatic lenses had been successfully constructed. 



In 1823, MM. Selligues and Chevalier in Prance, 

 Prauenhofer in Germany, Anna in Italy, and Dr. 

 Young and Mr. Tulley in London, turned their 

 attention to the construction of achromatic objec- 

 tives. M. Selligues was the first to make them of 

 four compound achromatic lenses. The focal length 

 of each combination was 18 lines (= H inch) ; the 

 diameter, 6 lines; the thickness in the centre, 6 

 lines; and the aperture, 1 line. This objective 

 could be used in combination or separate. 



Mr. Tulley at the same time constructed an 

 objective of -^ of an inch, and an angle of aperture 

 of 18°. This was probably the first English achro- 

 matic objective. Mr. Tulley afterwards added 

 another combination, increasing the angle of aper- 

 ture to 38°, To meet the requirements of the new 

 glass, the old form of stand had to be very much 

 modiGed. The construction of the new stand was 

 undertaken by Mr. James Smith, the optician. This 

 instrument had a mechanical stage, the first, it is 

 stated, ever made in England. 



(To be continued?) 



THE GLAUCOUS AND THE GREAT 

 BLACK-BACKED GULLS. 



By P. Q. Keegan, LL.D. 



A MONG the objects of interest which greet the 

 -*-•*- explorer of the Arctic Seas, there is none 

 more interesting than the magnificent sea-birds 

 whose names stand at the head of this paper. 

 Several of our Arctic voyagers bear testimony to 

 the freauent occurrence of the Glaucous Gull 



among the cliffs, headlands, or low-lying rocks 

 adjacent to various parts of the Polar seas. Prince 

 Regent's inlet, Baffin's Bay, and Davis' Straits 

 are especially mentioned as favourite localities for 

 the resort of this species. Away upon the loftiest 

 pinnacles of the icebergs which tower above the 

 surface of these sequestered seas, the bird may 

 frequently be discerned ; and invariably the trepi- 

 dation induced by the approach of a ship occasions 

 it to abandon its seat. 



The flight of the Glaucous Gull is regarded by 

 ornithologists as more buoyant than that of any 

 other species of gull. When its volatile apparatus 

 is completely expanded, it is observed to be lengthy, 

 powerful, and eminently competent to battle effec- 

 tively with the air. Its progress on the wing is 

 remarkably slow and stately, as if the bird was 

 under the influence of grave and solemn feelings. 



Owing to the character of its habits, the gull 

 now under review has been styled "the Burgo- 

 master of the Seas." Amid the dark atmosphere 

 of the northern climes, the spectacle of this bird 

 swooping down with tremendous emphasis and 

 impetuosity upon some unfortunate possessor of a 

 tempting edible morsel, is eminently calculated to 

 inspire a feeling of awe. The desolate solitude and 

 solemn silence of these regions, too, is frequently 

 disturbed by the fluttering wings and shrill, piercing 

 cries of this beautiful sea-bird. As may have been 

 anticipated, from the enlarged size, powerful build, 

 and predatory proclivities of this noble bird, the 

 quantity of food necessary to satiate its appetite is 

 by no means inconsiderable. Its voracity has been 

 frequently observed and commented upon by several 

 of our Arctic voyagers. The excessively cold re- 

 gions which it inhabits doubtless impart a tone and 

 sharpness to its appetite which birds dwelling in a 

 more temperate zone can never experience. In an 

 old work on Arctic zoology it is related, that the 

 Glaucous Gull "preys on dead whales, attends the 

 walruses in order to feed on their excrements, and 

 will even destroy and eat the young of the Razor- 

 bills. It aiso feeds on fish, and does not despise the 

 berries of the Empetrum nigrum." Dr. Richardson 

 also testifies to the fact, that one specimen killed on 

 Captain Ross's expedition disgorged an auk when 

 it was struck, and proved, on dissection, to have 

 another in its stomach. The bird now under review, 

 unlike the Divers, Guillemot, Gannet, &c, is firmly 

 and evenly seated upon its legs, thereby being 

 enabled to walk steadily and well. This circum- 

 stance is probably connected with its predatory, 

 carrion-devouring habits. By maintaining a firm 

 standing posture, it is capacitated to grasp, dissect, 

 and devour the flesh from the bones of a carcase 

 more effectually than it could possibly do if, like 

 some of the aforesaid birds, it were compelled, 

 whilst on shore to crawl along on its breast. 



The snecinc characteristics of the Glaucous Gull 



