WALKER-ARNOTT, ON ARACHNOIDISCUS. 159 
arm has been carried up to the highest point of the vertical 
stem, and the body of the microscope placed vertically, it 
may be used on the surface of the fluid of an aquarium 
standing two feet high; the upward and downward move- 
ments of the transverse arm being then used for fine adjust- 
ment. 
This arrangement was made for me by Messrs. Farquhar 
and Gill, plumbers and gas-fitters, of this city, for less than 
thirty shillmgs ; but it must be borne in mind, that for cheap- 
ness it is essential that selected pieces of ordinary brass 
tubing be made use of, that the vertical stem and the trans- 
verse arm be pieces of the same tube, and the pieces, g and 
h, parts of another tube; also, that the ball-and-socket joint 
be the one ordinarily used by gas-fitters. Many of the de- 
tails of the arrangement, especially those connected with the 
various pinching screws, may appear tedious in description, 
but it will be found that these are poimts of great conse- 
quence for securing comfort and facility in making any obser- 
vation, and therefore I have described them at length. The 
application of a good rack-and-pinion for the movement of 
the body of the microscope would be a valuable addition, but 
it would increase the expense considerably. 
On AracuNoipiscus. By G. Warxer-Arnort, LL.D. 
Before having a complete knowledge of the natural history 
of Diatoms, it is necessary that we know—lst, where, when, 
and by whom any object was first observed and brought 
under the notice of naturalists, whether by name, descrip- 
tion, or a figure: 2d, where, when, and by whom it was first 
correctly named and defined by a precise differential (generic 
or specific) character, the latter of these alone giving a 
claim of priority. 
It is not twenty years since the genus Arachnoidiscus was 
known. Short as that period is, I have not been able to trace 
its history with satisfaction. I shall, however, indicate here 
the information I have obtained, and hope that those connected 
with its discovery and description will complete the sketch 
before it be too late: already one (Dr. Bailey) has been 
removed. 
It is generally said, and I believe with justice, that 
