Cambridge PhilosopKical ^^cietp. 461 



m' iodide of potassium, and also a very small portion of bromide of 

 iron, or of iodide or bromide of arsenic. The image is developed by 

 protonitrate of iron, or by a solution of pyrogallic acid in acetic 

 acid and water, and fixed by a solution of hyposulphite of soda. 

 ^ By taking out the two first lenses of the collectors, the instrument 

 is; adapted for using sunlight. 



Note. — At the time that this communication was made to the 

 Society, Prof. Stokes had kindly made known to the author the re- 

 sults of his discoveries with regard to the rendering visible the che- 

 mical spectrum, but as he had not then made them public, the author 

 of this communication could not state the use that Prof. Stokes's 

 discovery enabled him to make of a screen composed of uranium 

 glass, or of infusion of horse-chestnut bark, for finding the focal di- 

 stance of the chemical image, or of arranging the lenses of the con- 

 denser so as to produce the maximum of chemical action. 



Also, since the communication was made, it has been found that 

 the instrument described gives light enough to impress an image on 

 any of the ordinary papers or Daguerreotype plates in periods ran- 

 ging between one and five minutes, with the oxyhydrogen and lime 

 light; and with direct sunshine the impression is almost instantane- 

 ous ; of course sunlight is much better than any artificial light when 

 it can be procured, both as regards speed and the clearness of the 

 picture produced. 



May 10. — Professor Miller gave an account of a new method of 

 adjusting the Knife-edges of a Balance. 



Also of a method of determining the height of clouds by night. 



May 24. — Professor Stokes gave a Lecture on the Internal and 

 Epipolic Dispersion of Light. 



Nov. 8. — Mr. Adams, F.R.S. &c., gave an account of some Tri- 

 gonometrical Operations to ascertain the difference of geographical 

 position between the Observatory of St. John's College and the 

 Cambridge Observatory. 



The observations, especially those of eclipses and occupations, 

 which were made during many years by the late Mr. Catton at the 

 Observatory of St. John's College, and which have recently been 

 reduced under the superintendence of the Astronomer Royal, render 

 it a matter of some importajice to determine the exact geographical 

 position of that Observatory. The simplest and most accurate means 

 of doing this aj^peared to be, to connect it trigonometrically with the 

 Cambridge Observatory. For this purpose, a base was measured 

 along the ridge of the roof of King's College Chapel, by means of two 

 deal rods terminated by brass studs, the exact lengths of which were 

 determined by comparison with a standard belonging to Professor 

 Miller. The extremities of the base were then connected by a tri- 

 angle, with a station on the roof of the Observatory at St. John's, 

 from which, as well as from the two former points, a signal post on 

 the roof of the Cambridge Observatory could be seen. The angles 

 at the extremities of the base, combined with the corresponding ones 

 at the station at St. John's, furnished two determinations of the 



