1863.] 537 



The skill evinced by Mr. Browning in the construction of this 

 instrument induced the author to have one made in which still 

 better effects might be produced, by multiplying the number of 

 prisms and increasing the magnifying power, with the necessary 

 precaution to avoid as much as possible loss of light. After a few 

 preliminary trials, it was finally arranged to use nine prisms, which 

 is the number that can be applied with this instrument, although 

 the arrangements are such as to allow the whole or any less number 

 to be used with the utmost facility. 



Verniers and micrometer screws are attached to the knife-edges 

 of the slit through which the light to be observed is admitted to 

 the collimator and to the telescope, also to the large circle of the 

 instrument ; these enable the observer to note the exact position of 

 the lines observed in the spectrum from whatever source it is ob- 

 tained, and thus enable him to repeat and verify previous results 

 with the utmost exactitude. 



When two small prisms, one refracting and the other reflecting, 

 are fixed outside the knife-edge slit, spectra obtained from three 

 separate sources can be simultaneously examined ; and an illuminated 

 micrometer scale enables the observer to note the precise relative 

 position of the lines in the three spectra without reference to or 

 reading off from the verniers. By this arrangement a most interest- 

 ing spectacle may be obtained, showing in the uppermost portion of 

 the field of view the spectrum of thallium, strontium, or lithium, 

 ignited in the flame of a Bunsen's gas-burner ; in the centre of the 

 field the spectrum of the same substance in the oxyhydrogen 

 blowpipe, and at the bottom one in the voltaic arc ; each successive 

 spectrum there exhibits an increased number of lines. 



With this spectroscope the author has ascertained that the green 

 line of thallium, so celebrated for its integrity, and hitherto believed 

 to coincide with one of the lines in the spectrum of baryta, does not 

 so coincide ; for by employing the nine prisms with a power of 80 on 

 the telescope, the thallium line is "clearly seen to occupy a dark space 

 in the baryta spectrum, close by the side of the bright line with 

 which it was supposed to coincide. 



A range of prisms is adapted to the telescope, the highest of which, 

 when used in conjunction with the amplifying lens, gives a power of 

 110 with good definition. 



