174 Intelligence and Miscellaneous Articles. 



The Blackdown silicified shells were next explained ; and it was 

 reasoned that the state of their silica offered arguments tending to 

 prove a passage of silica from the colloidal to the crystalline state. 



The second part of the paper discussed the Coprolites specially. 

 Their exterior appearance is extremely sponge-like, almost exactly 

 resembling some species of modern sponges. They are marked by 

 oscules of peculiar characters. 



The so-called " pores " of palaeontologists are well marked. Spi- 

 cules, triradiate, sexradiate, sinuous, defensive and connecting, 

 have been observed. They are siliceous in composition. On dis- 

 solving the coprolites in acid, the spicules are set free, associated 

 with Polycystina (Haliomma hexacaniha &c.) and Xantliidia (X. fur- 

 catum). The genera and species of coprolites described were as 

 follows : — Bhabdospongia communis, Bonneyia bacilliformis, B. cy- 

 lindricus, B. Jessoni, B» scrobiculatus, B. verrongiformis, Acantho- 

 pliora Hartogii, Polyacantha Etlieridgii, Retia simplex, M. costata, 

 Ulospongia patera, U. calyx, U. Brunii. The external appearance of 

 these forms, which constitute a vast number of the coprolites, their 

 curious oscules and siliceous spicules, were said to leave no doubt 

 as to their spongious origin. 



XIX. Intelligence and Miscellaneous Articles. 



A NEW METHOD FOR EXAMINING THE DIVISIONS OF GRADUATED 

 CIRCLES. BY G. QUINCKE. 



THE task of examining a graduated circle by which the position 

 of two telescopes, with microscopes for reading off, was to be de- 

 termined accurately to a few seconds, has led me to a method of 

 examination combining convenience and accuracy ; and, as far as I 

 am aware, it has not yet been described. 



The telescopes are provided with a Gauss's ocular (Astron. Nadir. 

 579. 31. 10, 1846), in which the cross-threads can be illuminated by 

 a plane glass placed between them and the lense of the ocular and 

 inclined at an angle of 45° to the axis of the telescope. 



The axis of the telescope is placed normal to a plane-parallel 

 glass plate, if the cross-wires coincide with the reflected image pro- 

 jected by the rays reflected from the plate. 



The glass plate can be set np normal to the graduated circle by 

 being fixed with wax upon a little turntable in its centre. 



The axis of the telescope and its axis of rotation are exactly 

 perpendicular to each other, if the cross-wires and their reflected 

 image coincide also after the telescope has been turned through 

 180°. With different positions of the plane glass firmly united to 

 the circle, we thus obtain also the excentricity of the axis of rota- 

 tion (if any exist) in relation to the centre of the circle. For the 

 sake of greater intensity of light I make use of Steinheil'3 plane- 

 parallel glasses, one side of which is silvered and polished. 



Two plane-parallel mirrors are fastened with wax upon the turn- 

 table, perpendicular to the axis of the telescope. They are exactly 



