424 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
influence of the wave-length on the resolving power of the apparatus is 
discussed. The fourth section deals with the development of the methods 
of illumination by reflected and transmitted light. The other sections 
are devoted to the improvements in negative and positive processes, 
treatment of preparations, &c. The book is illustrated with 61 wood- 
cuts, four autotypes, two photographic plates, and one photogravure. 
Photomicrography of the Solar Spectrum and Absorption Spectra.* 
— D. Joaquin Ma. de Castellarnau describes the method which he 
employs to obtain photomicrograms of the solar spectrum. In the dis- 
position of the Microscope and spectral eye-piece which he adopts, a ray 
of sunlight is directed by a Prazmowski heliostat upon a mirror by which 
it is reflected along the axis of the Microscope placed in the horizontal 
position. The Abbe condenser projects an image of the sun upon the 
plane of the stage ; the objective and field lens of the eye-piece form a 
second image in the plane 
of the diaphragm, and 
lastly the front lens of the 
eye-piece produces a third 
image on the ground glass 
of the camera. Between 
the front lens and camera 
is interposed a direct vision 
spectroscope. The eye- 
piece employed was a 
special projection one of 
Zeiss which consisted of 
two lenses giving a system 
perfectly aplanatic and 
achromatic. For taking 
absorption spectra the 
transparent solid or liquid 
under examination is 
placed on the stage of the 
Microscope. 
Portable Heliostat for 
Photomicrogr aphic Work. 
— This heliostat was ex- 
hibited by Mr. C. Lees 
Curties, at the meeting of 
November 18th last. It 
is made in the form sug- 
gested by Mr. T. Comber, 
and consists of a heavy 
brass base with levelling screws and spirit-level. A clock, rotating a 
spindle once in twenty-four hours, is hinged to the base. This spindle 
can be set to suit the latitude of any place between 15° and 70°, and 
carries a small adjustable first surface mirror. A second similar mirror, 
but larger, on an adjustable stand, is also fixed to the base, and if set 
at half the angle of latitude of the place, reflects a horizontal beam 
* Cronica Scientifica, Barcelona, August 1889, 7 pp. (sep. copy). 
