66 
Transactions of the Boyal Microscopical Society, 
and, acting as a field lens, also increases the amount of light. The 
result is that, although the dimensions of the instrument are so 
small, the dispersion, definition, and focal length for all parts of the 
spectrum are in every respect satisfactory for the examination of the 
class of objects for which it is designed. At the same time I must 
say that I should not adopt this construction when an increase in 
the size of the instrument is of no importance. The dispersion is 
diminished, and the definition is certainly not improved ; and we 
must be content if it is as good with an instrument half the size, 
since that makes all the difference between what is convenient and 
inconvenient for constant carriage in the pocket. In conclusion, I 
would say that I do not think it desirable to fix a reflecting prism 
over part of the slit of such spectroscopes, for the purpose of com- 
paring two spectra. This can never be done in a satisfactory 
manner with such an instrument, and the prism makes it longer, 
and prevents small objects from being brought sufficiently near to 
the slit. It should be looked upon as a very handy spectroscope, 
and not as a spectrometer. 
