OF THE CHARACTERISTICS OF THE DIATOMACEJS. 43 
confusion among authors, who give the most contradictory 
estimates of the numbers contained in 1*1000 inch. 
I think it is more than probable that, wheif the highest 
powers are being used, secondary representations, or images, 
of the stria? appear before the eye, owing to a possible dis- 
tortion of the rays of light, and that these images deceive 
the observer, and induce him to note down a larger numbef 
than really exists. 
The coarser lines may be measured by means of an 
ordinary stage micrometer ; but, for the finer structures, an 
eyepiece micrometer must also be employed. In either 
case, the operator must not be content with a single obser- 
vation of the stride, but must reckon them several times, so 
as to be able to draw an average, and get as near the truth 
as possible. 
As however the frustules, minute though they are, have 
a certain thickness sufficient to interfere with their tran- 
sparency, a special expedient must be resorted to to obviate 
this inconvenience ; otherwise the valvular markings can 
never be seen so clearly as is needful in the operations just 
spoken of. This may be managed by loosening the hold 
which the hoop, or outside frame, has on the two opposite 
valves, thereby setting them free, and enabling the observer 
to view each one as a separate object. 
To effect this, let the frustules be boiled in nitric acid, 
to which chlorate of potassium is to be added in small quan- 
tities. After the boiling has continued for about a minute, 
they must be well washed in soft water, in order to get rid 
of the acid. If this be carefully done, the intercellular 
Bubstance which binds the valves together is dissolved, and 
the latter fall apart. 
[For the various methods of mounting the Diatomacese in 
balsam and fluids the reader is referred to works specially 
devoted to the microscope. There is the less need to intro- 
duce the subject here, as it has been already exhausted in 
Mr. T. Davies’ admirable manual, 1 The Preparation and 
Mounting of Microscopic Objects.’ Hardwicke. — Ed.] 
E 
