350 ANNUAL OF SCIENTIFIC DISCOVERT. 



were to be obviated. In this actinic telescope how can we find tlie 

 tiHic focus? Only by a process of tentation wliich lie cxphiiniMl. 

 It has no power of accoraniodation, and reconls flian;;*''' <'t the 

 one-hundreikh of an inch, which the visual telescope cannot; nor 

 will any two actual observers ever agree to within the one-hun- 

 dredth of an inch on any focus. 



To show the advantages of this glass in stellar work, he showed 

 that it had taken images of stars of tht; nintii magnitude ; stars of 

 the sixth being the smallest before recfu-ded. He iiad begun to 

 work on the rit-iadcs, rx'ssel's wtu'k on this constcUation being so 

 accurate as to make it the Ijcst test of his own work. One plate, 

 exposed three or four minutes, gave him forty-three stars. On that, 

 Alcyone occupied twenty seconds, wliich was such an enormity 

 that it rcijuired examination, lie Ibuntl this cxtraordiMary breadth 

 to proceeil from her own light, increased by radiation witiiin the 

 tube of the instrument, and showeil how a bit of gauze nunoved 

 the dillicnlty. This breadth was iilk-d up by repeated impressions, 

 not by an instant's exposure and printing. The telescope follows 

 the star in a prolonged exposure ; if it is not quite prompt, it only 

 elongates the star in the direction of right ascension. He had at 

 first apprehended trouble from the shrinking of the collodion. It 

 had not come; if it did, he had still a resource in a certain tena- 

 cious varnish. Now, to ascertain the worth of his work, a microm- 

 eter was necessary. Xone in existence could be adapted to tiiis 

 instrument, so this also he must make himself. He described this 

 achievement, and showed a photograph of it. The extraordinary 

 accuracy of the work done by it we can only measure by the 

 amazement of the authorities present, who pressed eagei-ly about 

 him. He described his eye-piece with its bisecting lines. Ho be- 

 lieved that, henceforth, no observatory would be complete without 

 a recording glass. He showed, in conclusion, how he had forced 

 the light into the right direction within the tube by a supplemen- 

 tary lens. 



SECULAR INCREASE OF MEAN TEMPERATURE. 



Befoi'e the London Meteorological Society, Mr. Glaisher read a 

 communication on the " Secular Increase of Mean Temperature." 

 He stated that the mean temperature of the seven years ending 

 18(33 had been so high as to increase the mean temperature of the 

 3'ear from forty-three years' observations, viz., 48° 92' to 49° 04'. 

 He then remarked that the mean temperature of the first twenty- 

 five 5°ears ending 1838 was 48° (/, and of the twenty-five 3'ears 

 ending 18G3 was 49° 2'. The author then liecame desirous to see 

 if this increase had been progressive, and found the mean of 

 twentv-nine years ending 1799 was 47° 7', of thirty years ending 

 1829 was 48° 5', and ol" tliirty years ending 1S59 was 4;3° 0', 

 proving that the secular increase of the mean temperature was 2°. 

 This result he considered so important tliat he examined every 

 probable source of error, and concluded that no instrumental 

 errors would account for this increase. The questions he then 

 set himself to investigate were : Whether this increase had taken 

 place in every month in the year.'' or in some months or seasons 



