119 



This form of eye-piece gives a better correction of the oblique pen- 

 cils than the common negative. 



The time of exposure to obtain an intense negative six inches 

 diameter, on a collodion plate prepared as below, is about a minute ; 

 a positive is obtained in a fraction of a second. 



The collodion is formed by dissolving gun-cotton in sulphuric 

 aether, and adding to it a small portion of iodide of silver dissolved 

 in 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, 1852. 



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, 1852. 



Professor Stokes gave a Lecture on the Internal and Epipolic 

 Dispersion of Light. 



November 8, 1852. 



Mr. Adams, F.R.S. &c, gave an account of some Trigonometrical 

 Operations to ascertain the difference of geographical position lie- 



