157 
Vol. 3, 1835). The slide was carefully dried by heat, and 
the lines were as visible as before. Repeated observations 
appeared to prove that these faint lines really exist on the 
valves. There is little doubt but that the ordinary diagonal 
markings are on the outside, or convex surface of the diatoms 
and in the specimens under examination, the broken-up 
portions of the markings were scattered over the surface of 
the object. To account for the perfect preservation of the 
faint lines the author suggests that they are on the under, 
or concave, surface of the valve. The lines are visible by 
oblique illumination, or with the achromatic condensor and 
central stop. Microscopists are sometimes forgetful that 
thin and diaphanous objects have two sides, the author is 
reminded of a case in point : some thirty years back the 
scales of the Lepisma Saccharina were favourite test objects. 
The scales are very various in form, according to the position 
they occupy on the body of the insect ; some of the scales 
have a series of parallel lines running the whole length 
which are frequently crossed by lines radiating from the 
point where the scale is attached to the insect. At the 
period named the author was in the habit of frequently 
mounting Lepisma scales as objects for the microscope ; if 
too much pressure was applied in transferring the scales 
from the body of the insect to the glass slide, one set of these 
lines was rubbed off. It thus became clear that the parallel 
lines were on one surface and the radiating ones on the other. 
About the year 1844 the author exhibited the scales of the 
Lepisma in this condition, and gave this explanation to the 
late Sir David Brewster, who was on a visit to this town. 
Since then the late Mr. R. Beck corroborated this view of 
the Lepisma scale by means of the binocular microscope. 
It is quite possible that the markings seen on other diatoms 
than the Angulatum may not all be on one surface. The 
markings on diatoms, and especially those on the P. Angu- 
latum have been the subjects of much discussion amongst 
microscopists. At the present time differences of opinion 
exist on this point. 
