NOTES AND MEMORANDA. 
219 
Consequently, if the reflecting face of the crystal has its projection 
perpendicular to the zero line of the microscope, a reticle will be seen 
equally illuminated to the right and left ; but if the crystal is turned 
with the stage of the microscope, the reticle immediately becomes 
bordered on one side by a black band and on the other by a bright 
band. By placing the cube on the stage of the microscope, on its 
diflerent faces in succession, it is easy to measure the angles which 
the projections of the faces of the crystal make with the edges of the 
cube, and it is seen that, however small a crystal may be, the pheno- 
menon above described will be produced, provided that the crystal is 
able to reflect the light over a space sufliciently large to illuminate 
the centre of the reticle. It is sufficient that the face of the crystal 
should be about two millimetres, but a crystal of a thirtieth of a 
millimetre can be measured with an enlargement of only sixty dia- 
meters. A crystal of of a millimetre would require an enlarge- 
ment of two hundred times. 
In order to allow each face of the crystal to be placed successively 
in the axis of the microscope without changing the relative directions 
of the faces of the crystal, the edges of the cube, and the divisions of 
the stage, it is necessary to adapt to the revolving stage another stage 
movable in two rectangular directions by means of two micrometric 
screws, or, for more precision, to have a micrometric screw for the 
rotating movement. 
To appreciate the degree of precision which this method gives, I 
have measured crystals of less than of a millimetre, such as the 
cleavages of spar, of blende, and of the microscopic crystals of quartz, 
and the error has never exceeded one degree. 
This error is great, but these first attempts have been made with an 
eye-piece which is still imperfect, and I have not kept to the conditions 
of illumination which are necessary to obtain the best possible effect. 
I am convinced that, with slight modifications, great exactitude can be 
attained. 
I have in view another system of eye-piece, also based on total 
reflexion, which ought to be still more sensitive than the system 
which I have just described, but not having yet experimented upon it, 
I abstain from describing it. The improvements which I have in 
view will, if they answer, form the subject of a further communication.* 
The Microscopic Structure of the Stro^natoporidce, and on Palaeozoic 
Fossils mineralized with Silicates, in illustration of Eozoon. — Principal 
Dawson communicated a paper on this subject to the Geological 
Society, which was read at the meeting of the 5th June : — The fossils 
included in the group Stromatoporidae occur from the Upper 
Cambrian to the Upper Devonian inclusive, and are especially 
abundant in the Trenton, the Niagara, and Corniferous formations. 
The author regards Stromatopora as a calcareous, non-spicular body, 
composed of continuous, concentric, porous laminae, thickened with 
supplemental deposit, and connected by vertical pillars, most of 
which are solid. The surface shows no true oscula ; but perforations 
* ‘ Comptes Rendu-i,’ vol. Ixxxv. p. 172 . 
K 2 
