8 



Mr. W. H. M. Christie on the 



[Mar. 1, 



glands are in some way connected with the absorption of nitrogenous 

 matter from the putrid fluid in the cups, we are far from understanding 

 the whole of the problem ; for precisely similar jilament-protrudiiig 

 glands are found on the seedling leaves of D. sylvestris and on the second 

 year's leaves of B. jpilosus ; and as no " cups " are formed in either of these 

 cases, the filaments cannot be connected with absorption of the products 

 of decay. The only view which suggests itself is that the filaments 

 absorb ammonia from the dew and rain. Recent researches have shown 

 that certain leaves have the power of absorbing an appreciable quantity 

 of ammonia ; and this fact lends some probability to the view above 

 advanced. 



To recapitulate. Protoplasmic filaments are protruded from the leaf- 

 glands of the teasel ; and the only theory which seems at all capable of 

 connecting the observed facts is the following : — That the glands on the 

 teasel were aboriginally (£. e. in the ancestors of the Dipsacaceae) mere 

 resin-excreting organs ; that the protoplasm which comes forth was ori- 

 ginally a necessary concomitant of the secreted matters, but that, from 

 coming in contact with nitrogenous fluids, it became gradually adapted 

 to retain its vitality and to take on itself an absorptive function ; and 

 that this power, originally developed in relation to the ammonia in rain 

 and dew, was further developed in relation to the decaying fluid accu- 

 mulating within the connate leaves of the plant. 



III. " On the Magnifying-power of the Half-prism as a means of 

 obtaining great Dispersion, and on the General Theory of the 

 e Half-prism Spectroscope.' " By W. H. M. Christie, M.A., 

 Fellow of Trinity College, Cambridge. Communicated by 

 Dr. Huggins. Received January 25, 1877. 



On account of the oblique incidence of the rays on the isosceles prism 

 and the consequent diminution of the aperture of the collimator, a " half- 

 prism," formed by dividing an isosceles prism by a plane perpendicular to 

 the base, has frequently been employed for the commencement of the 

 train of prisms, and also for the end, though apparently without due con- 

 sideration of the effect of the "half -prism" on the dispersion of the other 

 prisms preceding in the train. This is a matter of some importance ; for 

 it will be found that when the angles of incidence and emergence are 

 unequal (as in the half -prism), the angular separation between two pencils 

 of parallel rays is increased or diminished according as the angle of 

 emergence is greater or less than the angle of incidence. In consequence 

 of this the angle between the pencils corresponding to any two lines in 

 the spectrum, e. g. the two D lines, will be increased by passing through a 

 half-prism (independently of the effect of ordinary dispersion) if the 

 perpendicular face be turned towards the slit. At the same time the 



