184 Mr. Gassiot on a Train of Eleven [April 7, 



Crown-glass prism. 



•t 



By this means the angle of minimum deviation of the prisms is so much 

 decreased, that eleven of them thus constructed can be used in a circle 

 instead of eight. An increase of dispersive power, due to refracting angles 

 of 150° of the bisulphide of carbon, is thus gained, minus only the small 

 amount of dispersion counteracted owing to the dispersive power of the 

 crown-glass prisms being employed in the contrary direction. 



From the well-known low dispersive power of this medium, however, 

 this loss is inconsiderable, amounting to scarcely more than a fifteenth of 

 the power gained. Owing to the minimum angle of deviation being 

 lowered, the further advantage is also secured of a larger field of view being 

 presented to the telescope by the first and last prism of the train. 



Each prism, in addition to the light metal frame referred to, 

 has a separate stand, furnished with 

 screws for adjusting the prisms, and 

 securing them at the angle of mini- 

 mum deviation for any particular ray. 

 The prism stands within a stirrup fur- 

 nished with a welled head. By this 

 arrangement the prisms can be removed 

 and replaced without touching their sides 

 — a matter of some importance, as all 

 fluid prisms show different results with 

 every change of temperature. 



For the sake of simplicity, the metal 

 framing of the prisms, and the various 

 adjusting-screws, have been omitted in [ y ' ' ' ' ' ^ 



the last sketch. 



The very unfavourable state of the weather prevented any observations 



* Direction of ray as it would pass through two pair of parallel sides, 

 t Direction of ray as altered by interposing the crown-glass prism. 



