Then DP, considered as equal to CL, will be to DS as 99,324 to 70,283.

And so the elevation of the point I by the refraction of this section is known.
[Illustration] 41.

Now let there be represented the other section through EF in the figure before the preceding one; and let CM_g_ be the semi-ellipse, considered in Articles 27 and 28, which is made by cutting a spheroidal wave having centre C.Let the point I, taken in this ellipse, be imagined again at the bottom of the Crystal; and let it be viewed by the refracted rays ICR, I_cr_, which go to the two eyes; CR and _cr_ being equally inclined to the surface of the crystal G_g_.
This being so, if one draws ID parallel to CM, which I suppose to be the refraction of the perpendicular ray incident at the point C, the distances DC, D_c_, will be equal, as is easy to see by that which has been demonstrated in Article 28.

Now it is certain that the point I should appear at S where the straight lines RC, _rc_, meet when prolonged; and that this point will fall in the line DP perpendicular to G_g_.

If one draws IP perpendicular to this DP, it will be the distance PS which will mark the apparent elevation of the point I.Let there be described on G_g_ a semicircle cutting CR at B, from which let BV be drawn perpendicular to G_g_; and let N to GC be the proportion of the refraction in this section, as in Article 28.