SPECTKO-MICROSCOPY. 



121 



spectroscope, of which the collimative-tube was inserted 

 in the body of the microscope, instead of an eye-piece. 

 With this apparatus he has succeeded in obtaining a 

 spectrum showing the absorption-bands from a mere frag- 

 ment of single blood-disc, when mounted as a transparent 

 object. In fig. 70, k represents the knife-edges, c the tube 

 containing the collimating-lens, P the prisms, T the teles- 

 cope, and m the micrometer ; the object is placed on the 

 stages at o, and must be illuminated from below if trans- 

 parent, or, if opaque, from above by any kind of con- 

 denser. 



Mr. Sorby suggested that a prism might be made of 

 dense flint-glass, of such a form that it could be used in 

 two different positions, and that in one it should give 

 twice the dispersion that it would in the other, but that 

 the angle made by the incident and emergent rays should 

 be the same in both positions. 



Fig. 71. 



Fig. 72. 



Figs. 71 and 72 represent prisms of the kind made by 

 Mr. Browning, used in two different positions, i and i' 

 being the same angle as I and i'. 



For most absorption-bands, particularly if faint, the 

 prism would be used in the first position, in which it 

 gives the least dispersion ; but when greater dispersion 

 is required, so as to separate some particular lines more 

 widely, or to show the spectra of the metals, or Fraun- 

 hofer's lines in the solar spectrum, then the prism must 

 be used as in fig. 72. This answers well for liquids or 

 transparent objects, but it is, of course, not applicable to 

 opaque objects. 



