Astronomy and Meteorology. 155 
which it was received by a small achromatic objective, and observed with 
an ey a sae magnifying twelve times, in the focus of which was placed 
a bar movable by a micrometer screw. 
my experience in the atc kt of stellar spectra, [-shou 
note several defects in this arr angement. Ist. The absence of achromatism 
in the great condenser, in te of peririe but a small portion of 
the spectrum can at any time be brought to ‘an approximate focus, and 
fine definition of the lines whee obtained. 
much power in the observing telescope, the objections to 
which are manifest. 
ad. The uncertainty of making a contact of the micrometer bar with 
the te — being no illumination. 
; waut of a che ck, such as the presence of a flame line in the 
feld of view, nn insure the detection : such small displacements of the 
The results of these Jastrutsental pie will I think be seen when we 
hereafter examine the ceeasgi sae 
T 
twelve inches aperture. He says “The pencil of light from the 
glass, which has converged to form the image of the star, then ite ges, 
and falls in a wide and divergent state upon the prism; after emergence 
it is received on a combinati ton of lenses which causes the pencils for the 
different colors to converge.” 
é image is a with -a micrometer, the field being illuminated 
by an annular reflector. In this form neither slit nor cylindrical lens is 
used, but breadth is oun to the spectrum of the star by the creation of 
aberrations in two ways; first, by placing the prism in a pesiios not 
that of least deviation, and second, by the uncorrected state of the “ com- 
ete of lenses” through which the hight reaches the micrometot after 
manuer: net err peti y a hole =}, of an inch in lant is 
ence is Saadeh’ with a aatiee in a Lice tabs and the position of the 
small hole observed. At night, the pe ae oa is removed, and the image 
of the star is made to occupy the same position when seen in the lateral 
eye-piece oceupied by the sun-illuminated hole, the eye-piece and re- 
fiector are then removed, and the observation made by the micrometer 
upon the star strize 
Mr. Airy, in his description, speaks of this form of spectroscope as = 
perimental, and expresses some doubts of its ability to define with 
sharpness, is defect must necessarily result from the construction of 
the i lustrument, since the lines are only rendered visible by the existence of 
ec tions, which are destructive of fine definition. The spectrum not 
being confined during the observation to any certain part of the field of 
4 view, . BY a slit or other check, the truth of the measures depends entirely 
re exact running of the equatorial driving clock, which is not to 
tah trusted, ‘The ijlumination of the field must necessarily obliterate the 
