328 Transactions of the Royal Microscopical Society. 
describe. I had meditated from the first employing quartz as 
being liarder and consequently more durable, but 1 wished to 
make use of glass if it were practicable. The next step was to 
grind three quartz prisms with great care and attention, and the 
result proved, I think, fairly satisfactory, and are now submitted 
for your verdict. 
It will, I think, be evident to everyone that if such a prism as 
I have described can without disadvantage be substituted for the 
usual slit, the narrow face of the prism may be very readily 
cleaned from any dust which may settle on it, by withdrawing the 
fitting and gently wiping it with a handkerchief or other suitably 
soft material. For want of a more expressive term, I have called 
this arrangement a ‘‘ solid slit.” 
It would perhaps be hazardous to state that it is quite a new 
feature in glass grinding to produce a surface of almost no width 
with nevertheless sharp edges, but at any rate I believe it is new, 
and there can be no doubt that with more practice the workmen 
would become more proficient in the production of such surfaces ; 
upon these, of course, the whole value of the arrangement rests. 
It is very pleasant to believe our own designs perfect, our ideas right, 
and our geese swans ; but if you do not accept this which is the 
main feature of my instrument, some of the minor alterations, which 
I will lay before you as briefly as possible, may meet with some 
approval. 
Presuming that this prism arrangement is condemned, you 
will see that I have here a slit arrangement, which slides in the 
same dovetail as the prism fitting. This is opened and closed by a 
micrometer screw from the outside, the head of which is divided 
for registration if necessary, the edge of the fitting carries an 
index, which slides past a silvered scale divided into lOOths of 
an inch. A better arrangement would have been to have a vernier 
instead of an index, and the readings then would have given 
thousandths, and the position of absorption bands or lines obtained 
by means of the bright spot above the direct vision prisms. There 
could be no more easy method, I think, for cleaning the jaws of a 
slit than by this method of making the fitting removable from the 
side. 
It is manifest that the distance between bands at the opposite 
extremes of the spectrum may be more rapidly measured by this 
sliding movement than by numerous turns of a micrometer screw ; 
such screw, I think, should only be used as a fine adjustment, and 
if it has such work thrown upon it as I have suggested, it would, 
I believe, be found to rapidly deteriorate. It may perhaps be 
argued that this sliding movement does not give such facility as a 
screw for accurately placing any line in exact juxtaposition with 
the point of the bright spot, but I feel sure that anyone would find 
