On the Illumination of small Arcs of the Horizon. 273 



€aptain Parsons, either dead or dying, through the long track 

 in which he observed the " gar fish ;" which we would account 

 for either from the peculiar floating properties that would be 

 possessed by a hard-skinned Sygnathus, or by the limitation 

 of the range of the destroying agent ; but although we cannot 

 distinctly account for this circumstance, the fact is of inte- 

 rest as showing the occurrence of apparently similar causes in 

 the destruction of the individuals forming the ancient fish- 

 beds, which are sometimes filled almost with one species only. 

 The accompanying figures, taken from Captain Parsons' log, 

 point out the track of the Harbinger in which the fish were seen. 



Lat 



K", 



Long. W. 



Lat. N. 



Long. "W. 







39° 



0' 



45° 56' 



22° 0' 



46° 



49' 



34 



10 



46 32 



20 30 



46 



24 



31 



45 



47 40 



19 35 



46 



22 



29 







47 56 



19 10 



46 



16 



27 



10 



48 34 



17 49 



46 



10 



26 







49 14 



15 47 



45 



59 



23 



46 



49 36 



13 17 



On a Simple Method of distributing naturally Diverging 

 Rays of Light over any azimuthal angle, with descrip- 

 tion of proposed Spherico-Cylindric and Double-Cylindric 

 Lenses, for use in Lighthouse Illumination. By Thomas 

 Stevenson, F.R.S.E., Civil Engineer. (With a Plate.) 



The diacatoptric apparatus of M. Augustin Fresnel is ad- 

 mirably adapted for the use of such fixed lights as require to 

 be constantly visible in every azimuth. There are, however, 

 many situations where only a small portion of the horizon re- 

 quires to be constantly illuminated, and yet that portion may still 

 be too great to admit of being lighted up by a single parabolic 

 reflector, the divergence of which does not exceed 15°. In such 

 cases the engineer has hitherto been contented, for want of more 

 perfect apparatus, either to employ a segment of M. Fresnel's 

 diacatoptric arrangement for fixed lights already referred to, 

 or to have recourse to several parabolic reflectors, each of which 

 requires its own separate lamp. When half the azimuth re- 

 quires to be illuminated, the use of one half of the diacatoptric 

 apparatus, having a spherical mirror behind, is perfectly legi- 

 timate, but where the arc to be illuminated is small, much light 

 would obviously be lost. A better effect would probably result 



