1870.J 



Physics. 



425 



found in the field of view. As a consequence of this peculiarity, 

 the violet and deep-red ends of the spectrum are greatly elongated, 

 or rather, much more of them can be seen than in an ordinary 

 spectroscope, and the H lines, which are generally seen only with 

 difficulty, come out in a marked manner. 



Drs. Roscoe and Thorpe have recently communicated to the 

 Royal Society the results of a series of determinations of the chemical 

 intensity of total daylight, made in the autumn of 1867, on the flat 

 plateau of the river Tagus, about &i miles south-east of Lisbon, 

 under a cloudless sky, with the object of ascertaining the relation 

 existing between the solar altitude and the chemical intensity of the 

 light. The experiments were made as follows: — 1. The chemical 

 action of total daylight was observed in the ordinary manner; 

 2. The chemical action of the diffused daylight was then observed, 

 by throwing on to the exposed paper the shadow of a small, 

 blackened, brass ball, placed at such a distance that its apparent 

 diameter, seen from the position of the paper, was slightly larger 

 than that of the sun's disk; 3. Observation No. 1 repeated; 

 4. Observation No. 2 repeated. Next, the means of observations 

 1 to 4 were taken. The sun's altitude was determined by a sextant 

 and artificial horizon. One of the sets of 134 observations was 

 made as nearly as possible every hour. It has been already pointed 

 out, and proved by experiments made at Kew, that the mean 

 chemical intensity of total daylight, for the hours equidistant from 

 noon, is constant. The results of the present series of experiments 

 prove that this conclusion holds good generally. One of the chief 

 results arrived at is that, although the chemical intensity for the 

 same altitude, at different places and at different times of the year, 

 varies according to the varying transparency of the atmosphere, yet 

 the relation, at the same place, between altitude and intensity, is 

 always represented by a straight line. 



A new and very ingenious graduating diaphragm for the micro- 

 scope has been contrived by Mr. J. Zentmayer. This exceedingly 

 ingenious arrangement is shown in the accompanying cuts, which 

 are taken from photographs ; Fig. 1 showing the apparatus with 



Fig. 1. 



Fig. 2. 



its largest, and Fig. 2 with its smallest opening. To obtain a 

 circular diaphragm which, like the eye, should expand and contract 



