1814.] A Treatise m New Philosophical Instruments, 461 



action of a great variety of transparent bodies upon the differently 

 coloured rays of light ; but owing to the extreme delic;icy of the 

 results, he did iiot'attenipt to express them in numbers, though he 

 has pointed out a method by vviiich this may be effected. The 

 general results of these experiments are given in the following table, 

 which shows the order of the substances that form spectra, in which 

 the red and green spaces are most contracted, and the blue and 

 violet ones most expanded : that is, the substances are arranged 

 inversely according to their action upon green light : — 



Oil of cassia. Oil of lavender, Ether, 



Sulphur, Canada balsam, Leucite, 



Balsam of ToUi, Oil of turpentine. Blue topaz. 



Carbonate of lead, Flint ghiss, Fluor spar. 



Oil of aniseeds, Calcareous spar. Nitrous acid. 



Oil of sassafras, Oil of almonds. Muriatic acid. 



Opal coloured glass. Crown glass. Rock cr\stal. 



Oil of cummin, Gum arabic. Water, 



Oil of cloves, Alcohol, Sulphuric acid. 



From these results it follows that the action of transparent bodie* 

 on the green rays in geverul diminishes as their dispersive power 

 increases, and that the" sulphuric acid exceeds all transparent sub- 

 stances, that have yet been examined, in its action upon the green 

 rays, while oil of cassia exerts the least action upon them of any 

 known substance. 



In applying these conclusions to the improvement of the achro- 

 matic telescope, the author lays down the following maxims :— 

 1. The practical optician should always select flint glass with the 

 least dispersive power. 2. The difference between the dispersive 

 powers of the crown and flint glass should be as small as possible. 

 By attending to these points the uncorrected colour will be the least 

 possible, and the performance of the instrument greatly improved. 



In the course of these experiments Dr. B. discovered what he 

 calls a leiliary spednim, produced by varying the inclination of the 

 first prism to the incident rays. This new spectrum appears even 

 when the two prisms are formed out of the same substance. 



The instruments described in the remaining chapters of this book 



are, 



1 . A new compound microscope for examining objects of natural 

 history, and cajjable of being rendered achromatic. 



2. A new polar microscope, which can be rendered achromatic. 



3. New fluid microscopes. 



4. Adjusting microscopes, for seeing at two different distances at 

 the same time. ^ 



5. Opera glasses and night glasses upon a new construction. 



