438 ON A MONOCHROMATIC LAMP FOR MICROSCOPES, &C 



Independent of its use in microscopical observations, the 

 Monochromatic Lamp will find an extensive application in va- 

 rious branches of the arts and sciences. In certain cases of 

 imperfect vision, where a number of coloured images are 

 formed by the separation of the fibres of the crystalline lens, 

 a homogeneous light will improve the vision, by removing 

 the prismatic tints, which obliterate the principal image. In 

 illuminating the wires of Transit Instruments and Micro- 

 meters ; — in graduating the limbs of divided Instruments, 

 which is generally done by candle-light ; — in reading off the 

 same divisions in fixed observatories ; — in forming signals in 

 Trigonometrical Surveys j — in obtaining correct and uniform 

 measures of Refractive Powers ; — in measuring the separation 

 of the two pencils in Doubly-refracting Crystals ; — in deter- 

 mining the focal lengths of lenses; — in observing various op- 

 tical phenomena, where the light is decomposed ; — in these, 

 and, in general, in all delicate works, where correct vision is 

 essential, the employment of a homogeneous flame will be 

 found to confer the most signal benefits. 



Having thus described the construction and application of the 

 Monochromatic Lamp, I shall now proceed to give a short ac- 

 count of the experiments which I was led to make, during this 

 enquiry, on the modification of the prismatic spectrum by the 

 action of differently coloured media. 



These experiments relate to three different points : 

 \st, To the manner in which coloured media absorb the dif- 

 ferent portions of the prismatic spectrum. 

 2d, To the influence of heat in modifying this absorbent 



power ; and, 

 3d, To the determination of the question, whether or not 

 yellow light has a separate and independent existence 

 in the solar rays. 



1-Dc 



