174 Mr. M. Ponton on certain Laws 
From a considerable number of trials made with various sets 
of normals, it appears that any alteration on the above numbers, 
within the probable limits of error, would not affect the general 
character of the laws above indicated. 
From the foregoing investigation it follows that the refractive 
index deduced from observation for any of the fixed lines, is a 
somewhat complex quantity. In the first place, each index in- 
volves a certain fixed amount ¢, which is constant for waves of 
every length. Itis the common divisor by which the differences 
between the normal wave-lengths would have to be divided, in 
order to produce within the medium a set of wave-lengths which 
should present no extrusion of the fixed lines, but in which each 
line should occupy the same position in relation to the others as 
the normal lines occupy in the spectrum produced by transmit- 
ting a divergent beam through a system of equidistant fine lines. 
This quantity ¢ is always less than the observed refractive index ; 
so that it forms only a portion, yet by much the greater propor- 
tion of its amount. Had all the fixed lines this constant ¢ as 
their common refractive index; the medium would then have 
refractive power without either dispersive or extrusive power. 
Although no single medium exhibits this peculiarity, it is pos- 
sible by a combination of two or more substances to obtain a 
compound medium that shall present this condition, which is 
that of achromatic refraction. The preceding investigation, if 
followed out, may tend to facilitate the effecting of such com- 
binations. 
As the purely refractive power of a medium is due simply to 
the state of compression of the ether within its pores, the con- 
stant « may be viewed as the measure of that compression, and 
may accordingly be termed the compressive index of the medium, 
as distinct from the refractive index, which is a complex quan- 
tity ; or it might be termed the optical elasticity of the medium, 
for it is at least a measure of that elasticity. In doubly-refract- 
ing media the value of e differs considerably in the direction of 
the different optic axes of the same medium, thus showing the 
‘compressive power of the constituent molecules to be specific and 
polarized. 
The variety in the refractive indices of the fixed lines in any 
medium is due primarily to its dispersive power. The effects of 
this property, viewed apart from the extrusive power, are exhi- 
bited by the series of indices “B,C, &c. obtained from the 
U : if 
formulee > t= and 7 Me in which only the normals and 
the two constants e and aareinvolved. The differences between 
this series of indices and the constant e may accordingly be 
viewed as the respective dispersive indices of the fixed lines in 
