12 METHODS OF PETROGRAPHIC-MICROSCOPIC RESEARCH. 
(b) The draw tube to which are attached the objective, the ocular, the 
Bertrand lens, the analyzer, and an iris diafram, and into which, at M , cer- 
tain optical wedges and plates can be inserted. 
(c) The stage 0, a rotating and mechanical platform upon which the 
object to be examined is placed. 
(d) The substage, which contains the condenser, an iris diafram, and 
occasionally other devices for illuminating the object through the central 
aperture of the stage. 
For satisfactory work, it is essential that both the mechanical and the 
optical systems fulfill certain requirements which are postulated by the 
fundamental principle of construction that the scientific observer, in any 
line of research, should have definite control over the different parts of his 
instrument, and should be in a position to adjust and to test the adjustment 
of each part, if his observations are to be free from unknown factors which 
may seriously affect the accuracy of the final result. For this reason each 
scientific instrument should be designed with reference to its adjustment 
and to the testing of the same. Simple adjustment facilities should be in- 
cluded on the instrument and a definite order of procedure for the adjust- 
ment given which the observer may follow step by step with the assurance 
that each successive step in the adjustment will not disturb the preceding 
step and thus necessitate a fresh start from the beginning. Unfortunately 
the importance of adjustment facilities is often disregarded by makers of 
scientific instruments, and the observer is compelled either to devote many 
hours or days to proper adjustment tests or to trust blindly to the adjustment 
by an unknown mechanic and to assume that the instrument has continued 
in adjustment notwithstanding the gradual seasoning of the whole and the 
vicissitudes of transportation and rough handling. In accurate work such 
conditions are obviously intolerable. Good adjustment facilities are as 
essential in a scientific instrument as good construction; by their use the 
observer is able not only to detect and to determine accurately the various 
instrumental errors which may occur, but often also to decrease appreciably 
the effect of these on the final result, and thus greatly to increase the accu- 
racy of his observations. 
In the petrographic microscope the adjustments are relatively simple and 
can be made without elaborate apparatus ; for this reason the importance 
of proper adjustment is often disregarded by the petrologist who accepts 
the instrument as he finds it and allows unknown factors to creep into his 
observations on the assumption that these are negligible in their effect and 
can be disregarded. This is unfortunate, as it introduces anelementof uncer- 
tainty into his results and destroys their intrinsic value to just that extent. 
In the mechanical construction of the petrographic microscope the follow- 
ing requirements are usually considered important : 
(1) Firm, rigid stand for the support of the optical system. 
(2) Optical system centered ; optic axis of the system to pass through 
the center of rotation of the stage. 
(3) Simple device for centering the objective ; the centering screws to 
be parallel with, and not diagonal to, the cross-hairs of the ocular 
in order that the observer may have field coordinates as guides. 
To center the stage instead of the objective is wrong in principle 
as it displaces the one point to which the optical system is tied. 
