22 THE ARSOKITION AND EMT8SI0X OF A IK. 



slit, say for a slit of width 0.001 mm. ; aud it is proposed to photograpli the .same 

 line with unaltered setting, but with a slit of width 0.005 mm.; then undoubtedly 

 that border of the line which corresponds to the stationary edge of the slit would 

 remain in focus, and retain its original sharpness, while the other, since the altered 

 position of the slit-edge corresponds to a different focus, and its image coii.se(piently 

 no longer falls within the sensitive film, would become less sharp, and the less so, 

 the more the slit is widened. A good photograph, in short, can now only be ob- 

 tained after a new focussing has rendered the two edges of the line equally hazy. 



Of course, as far as resolution is concerned, photogi-ajilis taken with a wide slit 

 are greatly inferior to those with fine slits. AVhen the fineness of single lines is 

 desired, as in the measurement of wave-lengths, or the resolution of dense groups 

 of lines, the slit must be nariowed to its utmost. In such cases, I have gone 

 down from several microns to a single micron. Good results of this sort can only 

 be e.xpected with faultlessly linear and sharp-edged slit-jaws. Rounded slit-edges 

 and those formed l)y little planes, such as are found in the majority of spectrome- 

 ters, ai-e not adapted to the i)urpose, since the image of the slit is broadened by the 

 light reflected from the sides, and conse(juently their I'esolving powei- is greatly 

 injured. The duration of exposure and that of development must, if the desireil 

 fineness and sharpness of the image is to be attained, be scrupulously adapted to 

 this purpose. Otherwise, the line will be too thin or broadened by diffraction to 

 utter worth lessness. But with some painstaking one soon succeeds in photograph- 

 ing lines so fine that they are oidy formed of one or two rows of grains of silvei- 

 somewhat coarser than the average and, it must be admitted, not quite regularly 

 distributed. The hydrogen spectium shown affoids some examples of this kind. 

 But let it be expressly noted that the si)ectrum, with all its lines, can never 

 be successfully photographed with slits of 1 /< or 2//, .supposing that it contains 

 many thickly crowded lines of very different intensity. The stronger lines, with a 

 time of exposure .suited to the weaker ones, acquire great broadening, giving them 

 a washed-out look unadapted to measurement. This is my reason for photographing 

 every one of the fields in the spectrum of hydrogen four times over with incieasinf 

 time of exposuie and sometimes with widened slit. This proceeding is the best 

 adapted for cai-ryiug the resolving effect of a vacuum-spectroscope of such slight 

 dispersive power to its highest. If we compaie the dispei*sion of this })rism-spec- 

 trograph in the most refrangible ultra-violet with that of a gratiug-specti'ograph, the 

 latter will tui'n out, notwithstanding its considerably longer focus, but moderately 

 superior. A numerical example will best sliow this. In the vacuumspectrograph, 

 when the aluminum lines at 1989.90 and 1854.09 Angstrom units (focal length in 

 vacuo 106.75 mm.), with a 60° prism appear at the middle of the plate, they 

 are separated by a distance of 5.8 nun. from one another, while in the spectrum of 

 the first order of a grating spectroscope belonging to me (having a concave grating of 

 106 cm. radius and 1-1,-131» lines to the inch), they are separated by 8.2 mm. as meas- 

 ured on Rowland's circle. With increasing refrangibility this ratio varies more 

 and more to the advantage of the prismatic spectrum, so that for the shortest wave- 

 lengths it is superior in dispersion to the grating spectrum of the first order. 



