ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



353 



to the two diagonals of the squares, just as would arise from sets of 

 lines in the direction of the diagonals, so that if the theory holds good 

 we ought to find, on obstructing all the other spectra and allowing 



Fig. 103. 



Fig. 104. 



only the diagonal ones to pass to the eye-piece, that the vertical and 

 horizontal lines have disappeared, and two new sets of lines at right 

 angles to the diagonals in their place. 



On inserting the diaphragm, Fig. 103, and replacing the eye-piece, 

 we find, in the place of the old network, the one shown in Fig. 104, the 



squares being, however, smaller in the proportion of 1 : v 2, as they 

 should be in exact accordance with theory. 



An object such as Pleurosigma angulatum, which gives six difixac- 

 tion spectra arranged as in Fig. 105, should according to theory show 

 markings in a hexagonal arrangement. For there will be one set 

 of lines at right angles to h a e, another set at right angles to c af, 

 and a third at right angles to g a d. These three sets of lines will 

 obviously produce the appearance of fields arranged as shown in 

 Fig. 106. The exact shape of the bright spot of every field is shown 

 by theory to be circular, which is readily verified by observation. 



Ftg. 105. 



Fig. 106. 



A great variety of other appearances may be produced with this 

 same arrangement of spectra. Any two adjacent spectra with the 

 central beam (as 6 c a), will form equilateral triangles, and give 

 hexagonal markings. Or by stopping ofi^ all but gee (or h df) we again 

 have the spectra in the form of equilateral triangles ; but as they are 

 now further apart, the sides of the triangles in the two cases being as 

 V^: 1, the hexagons will be smaller and three times as numerous. 

 Their sides will also be arranged at a different angle to those of the 

 first set. The hexagons may also be entirely obliterated by admitting 

 only the spectra g c or gf or h f, &c., when new lines will appear 

 parallel, at right angles, or obliquely inclined to the median line. 



