88 REPORT— 1840. 
minute differences of focal length, which is easily accomplished by a 
scale and vernier, reading to the thousandths of an inch. 
Thirteen minerals were cut with parallel planes, all exactly 5th of 
an inch thick; they were placed on the glass micrometer, focused in 
every experiment at *210 upon the scale; the difference of focus is there- 
fore the remainder of ‘210 subtracted from the figures cpposite the 
name of each mineral in the annexed list. 
Greenockite, Bishopton, Clyde .............. ‘275 
(Genanias) (Gesu lon 3 Anes sO boon eg eos +260 
ATELY SC POTION EAL ty -)-\ ocho were eiera ete ole siw/e aieieete is "258 
Pxehimitesgisislion fon tee ae cies esatete’eters) eke ctelsi crores *258 
Topaz Newitollaid’\!. oF raved i ates "255 
ERA DEBS UC: Blo eee ae CEE ae ates se i "255 
Sul platelets lan yitey ise e512, 5 ots nies 60: <15/a)s180) 912 °255 
Beryl, planes perpendicular to axis .......... °255 
Opaleconmovoitger emia de <p) classes sic sieaic cisalaistee 245 
Rose quartz .... 2.050 aetna cicba atta aceite tiers *246 
PAMICTHU SE CESEA AEC yz, .fe/atc, tone. tie areisiaiae a alts occis © 245 
HIGOGR PAM eh ciate ciswicemeecleeletwitetia tei wiceelete 243 
AEPAPOMIEOY Pais sie siete «ee oe Sea tars ctoiets eiemramiele 235 
On a New Method of Crystallographic Notation. By Mr. J. J. 
GRIFFIN. 
* 
The author classes the planes of crystals into seven elementary sets, 
which he calls “ Forms.” The planes of crystallized minerals consist 
of these forms in various states of combination. Hence a natural 
crystal, speaking crystallographically, is a “ Combination.” Theseven 
fundamental forms are named P, M, T, MT, PM, PT, PMT. These 
symbols show the relations of the planes which constitute the Forms, 
to what are termed the axes of acrystal. These axes are three mathe- 
matical lines which cross one another in the centre of the crystal, at 
an angle of 90°. The position of the first of these axes is perpen- 
dicular, whence it is called the principal or perpendicular axis, and is 
denoted by the sign p*. The second axis iS called the minor or middle 
axis, and is denoted by the sign m*. It passes from the front to the 
back of the crystal. The third axis is the transverse axis. It passes 
from the left to the right side of the crystal, and is denoted by the sign 
t®, All the planes of a crystal, when extended, cut one or two or 
three of these axes, and they are denoted by letters referring to the 
axes which they cut. ‘Thus P means two planes that cut p*; M, two 
planes that cut m?; T, two planes that cut t?; MT, four planes that 
cut m* and t*; PM, four planes that cut p* and m?; PT, four planes 
that cut p® and t®; PMT, eight planes that cut all the three axes. 
When the axes are cut at different distances from the centre of the 
erystal, the lengths of the respective axes are indicated by indices 
placed between the letters, which constitute the symbol of the form. 
Thus, M}T denotes a vertical rhombic prism, the diagonals of whose 
cross section are as the numbers 1 and 2. And P3M3T denotes a 
