THE PETROGRAPHIC MICROSCOPE. 13 
(4) Easy passage from parallel to convergent polarized light. 
(5) Easy passage from low to high powers. 
(6) Bertrand lens centered and adjusted to proper focus. 
(7) Properly constructed coarse and fine adjustment screws for focus- 
sing the objective, the fine adjustment screws to record intervals 
of o.ooi mm. and to be free from lost motion. 
(8) Satisfactory arrangement for raising and lowering the substage 
condenser. 
(9) Accurately constructed mechanical stage on which lateral move- 
ments of o.oi mm. can be measured directly. 
(10) Degree circle of stage to be accurately divided and provided with 
vernier to read to 5' at least. 
(n) The ocular, the upper nicol carriage, the Bertrand lens support 
in short, all moving parts to fit accurately, so that on insertion 
they invariably return to exactly the same point. 
Mechanically, the petrographic microscope is judged not only by the 
above criteria, but also by the ease and rapidity with which its different 
parts can be manipulated and by the rigidity and wearing qualities of the 
whole. The external finish of the instrument, although important from the 
standpoint of the sales agent, is secondary to the above considerations. 
The optical system of the microscope consists of several different working 
parts, each one of which has a definite function to fulfill and is constructed 
with that end in view. The calculation of these component lens systems is 
an extremely complex affair and requires long training and experience to 
accomplish satisfactorily. The minute objects in the thin section are often 
highly magnified by the microscope and the slightest defect in its design or 
construction is at once felt in the inferior quality of the image produced. 
Fortunately for the petrologist the detailed knowledge necessary to design 
the optical system of a modern microscope is not essential to good work with 
the completed instrument and will not be considered even in outline in this 
paper. The functions of each part of the instrument, however, should be 
understood, in a general way at least, if accurate results are to be obtained 
on fine-grained preparations. 
To facilitate the presentation of this part of the subject, which is some- 
what involved, a few introductory paragraphs, recalling several important 
definitions and the simple lens formulas, may well be inserted at this 
point, especially as constant reference will be made to them in the pages to 
follow. 
