224 Analyses of Books. [Szrr. 
there will be two pairs of images formed, and these may fall in 
any direction with respect to each other; but turning the move- 
able tube with the prism round more or less will bring the four 
luminous images into one straight line ; in which position, if the 
second and third images coincide exactly, the measure will at 
once be correct; but if not, the distance between the lenses 
must be varied until this coincidence takes place. Should the 
prism used be found to have too great or too small an angle at 
any of the distances marked in the scale, it must be changed for 
another having a more suitable angle, and must be adjusted as 
before directed. In all cases where one of two contiguous stars . 
is much smaller in appearance than the other, and is yet visible 
through the prism, the small one will be lost by super-position on 
the larger, and must, therefore, be made to pass over its centre 
by a slow motion given by rotation of the tube, when an esti- 
mate may be made of the exactness of the central transit ; or 
otherwise, the four visible images may be formed into an exact 
square, when it will appear whether or not the bounding sides of 
the figure are equal to each other; and if they are, the proper 
distance will be indicated in that position.” These directions 
are illustrated by a tabular account of the actual application of 
the micrometrical eye-piece to various celestial measurements ; 
and the memoir concludes with extensive tables of powers and 
measures, of which. it is not possible to give an intelligible 
abridgement. 
Of the sixth memoir, ‘‘ On the Construction of a New Posi- 
tion-Micrometer, depending on the Doubly-refractive Power of 
Rock Crystal,” a very short notice will be sufficient. Before 
proceeding to describe the addition made to the former instru- 
ment, a method is mentioned by which Dr. Pearson varies the 
constant angle of a prismatic solid, by the juxta-position of a 
second solid of double refraction ; a method which, says he, 
“to me is new, but which probably may be known to those 
philosophers, who have studied more minutely the laws of the 
polarization of light.” By what arrangement the eye-piece 
micrometer with double images is converted into a position- 
micrometer, the following extract will render sufficiently obvious. 
“When a crystal of the micrometer was applied before the eye- 
piece of a transit instrument, all the spider’s lines, as was 
expected, were seen double ; as was also a star or other luminous 
point placed at a distance. But turning the prism round a little, 
soon brought all the images of the vertical lines into contact 
with the lines themselves, and the coincidence was perfect as to 
breadth, but not as to length of the lines in question: the image 
of the star in the mean time revolved round the star itself with- 
out coming into contact. Likewise when two stars, in the same 
field of view, are examined through a doubly refracting prism, a 
line connecting either star and its own image will be truly verti- 
cal, when the image of the vertical line is coincident with the 
