Chemistry and Physics. 483 



hopes that some one having the use of a telescope of sufficient 



size may undertake such measurements. A plate ruled with 



squares is inserted in the eye-piece of . -^ . -> 



a minute circle of collodion near the center as in the ph 



of Dove. It is illuminated in front by the nebula, and behind by 



a plate of glass inclined at an angle of 45° which r 



; whose width may be varied and measured, or in other 

 To measure the brightness of a nebula the various portions are 

 brought in succession into the center of the field and the light 

 varied until the spot disappears. The exact position of each point 

 is found by observing the various positions of any star in the 

 field with regard to the squares. The real motion of the photom- 

 eter is thus found from the apparent motion of the star, A con- 

 tour map may then be constructed showing the brightness of the 

 various portions, and would soon show any marked changes in 

 the light of the various parts. The light of the adjacent sky 

 must be similarly measured and subtracted from all the read- 

 ings. The brightness may be compared with that of any star by 

 throwing the latter out of focus until its disk attains a given size, 

 and a star photometer is thus obtained. Observations on a comet, 

 with contours showing its brightness on various days, would be 

 both interesting and valuable. The brightness of different por- 

 tions of the moon could be measured by slightly modifying this 

 photometer. By using a very low power the light of an aurora, 

 of the zodiacal light or of different portions of the sky could be 

 similarly measured. For very faint objects it might be better to 

 insert a diaphragm in the eye-piece having an aperture but little 

 larger than the collodion film, thus giving a dark background. 

 Positions could then be determined by the finder or by moving 

 the entire eye-piece by micrometer screw^s. 



SCIENTIFIC INTELLIGENCE. 



I. Chemistry and Physics. 



1. On Hydrocellulose. — In some of the processes in the arts 

 which cellulose is used, as in paper-making, this substance und( 

 goes a transformation by which it is rendered friable. Ai.v 

 GiRARD has investigated the matter, and finds that this change 

 owing to the assumption of a molecule of water by the cellulose 

 form a new body of the composition, Ci^lIooO,,, to which 

 gives the name of hydrocellulose. To prcprm' it, sr.mi' iv>rni 

 purified cellulose, such as carded cotton, is phiceil in sul])hnri<' :ic 

 of 45° B. in the cold for twelve hours. It i> then \v.Il wn-hi 



