the ellipse is to be drawn. When pm is greater than pa the
ellipse touches the circle in two points; these points divide the
ellipse into two parts, one of which, being on the other side of
the meridian plane aqr, is invisible. Fig. 6 shows the complete
orthographic projection.-Slereographtc
Projection.-In this case the point of vision is
k I . .

on the surface, and the projection IS ° made on the plane of the great circle L whose pole is V. Let kplV (fig. 7) be a A great circle through the point of vision, and ors the trace of the plane of projection. Let c be the centre of a small

circle whose radius is cp=cl; the straight line pl represents this small circle in Fl; 7 orthographic projection.

We have first to show that the stereographic projection of the small circle pl is itself a circle; that is to say, a straight line through V, moving along the circumference of pl, traces a circle on the plane of projection ors. This line generates an oblique cone standing on a circular base, its axis being cV (since the angle pVc =angle cVl); this cone is divided symmetrically by the plane of the great circle kpl, and also by the plane which passes through the axis Vc, perpendicular to the plane kpl. Now Vr-Vp, being =Vo sec kVp-Vk cos kVp =Vo-Vk, is equal to Vs-Vl; therefore the triangles Vrs, Vlp are similar, and it follows that the section of the cone by the plane rs is similar to the section by the plane pl. But the latter is a circle, hence also the projection is a circle; and since the representation of every infinitely small circle on the surface is itself a circle, it follows that in this projection the representation of small parts is strictly similar. Another inference is that the angle in which two lines on the sphere intersect is represented by the same angle in the projection. This may otherwise be proved by means of fig. 8, where Vok is the diameter of the stphere passing through the

point o vision, fgh the plane of

projection, kt a great circle, passing of course through V, and ouv the

line of intersection of these two

planes. A tangent plane to the

surface at t cuts the plane of projection in the line rvs perpendicular

to av; tv isatangent to the circle kt at t, tr and ts are any two tangents to the surface at t. Now the angle

vtu (u being the projection of t) is 90° - otV= 90° - oVt= ouV= tuv,

therefore tv is equal to uv; and

since tvs and uvs are right angles, it follows that the angles vts and vus are equal. Hence the angle rts also is equal to its projection rus; that is, any angle formed by two intersecting lines on the surface is truly represented in the stereographic projection.

In this projection, therefore, angles are correctly represented and every small triangle is represented by a similar triangle. Projections having this property of similar representation of small parts are called orthomorphic, conform or conformable. The word orthomorphic, which was introduced by Germain* and adopted by Craig,2 is perhaps the best to use. Since in orthomorphic projections very small hgures are correctly represented, it follows that the scale is the same in all directions round a point in its immediate neighbourhood, and orthomorphic projections may be defined as possessing this property. There are many other orthomorphic projections, of which the best known is Mercator's. These are described below. Y

We have seen that the stereographic projection of any circle of the sphere is itself a circle. But in the case in which the circle to be projected passes through V, the projection becomes, for a great circle, a line through the centre of the sphere; otherwise, a line anywhere. It follows that meridians and parallels are represented in a projection on the horizon of any place by two systems of orthogonally cutting circles, one system passing through two hxed points, namely, the poles; and the projected meridians as they pass through the poles show the proper differences of longitude.

To construct a stereographic projection of the sphere on the horizon of a given place. Draw the circle vlkr (fig. 9) with the diameters 1 A. Germain, Traité des Projections (Paris, 1865). T. Craig. A Treatise on Projections (U.S. Coast and Geodetic Survey, Washington, xS82).

~

s o

FIG.8.

kv, lr at right angles; the latter is meridian. Take koP equal to the

co-latitude of the given place, say u; draw the diameter PoP1, and vP,

vP' cutting lr in pp': these are the projections of the poles, through which all the circles representing meridians have to pass. All their centres then will be in a line srnn which crosses pp at right angles through its middle

point rn. Now to describe the meridian whose west longitude is w, draw pn

making the angle opn=9o°-w, then

n is the centre of the required circle, whose direction as it passes through p will make an angle opg=w with pp. I The lengths of the several lines are FIG. 9. to represent the central

mn.

IV

op =tan%u; op'=cotén; om=cotu; mn=cosec n cot w. Again, for the parallels, take Pb =Pc equal to the co-latitude, say c, of the parallel to be projected; join vb, vc cutting lr in e, d. Then ed is the diameter of the circle which is the required projection; its centre is of course the middle point of ed, and the lengths of the lines are

od =tané(n-c); oe =tan% (u -1-c).

The line sn itself is the projection of a parallel, namely, that of which the co-latitude c = 180° -n, a parallel which passes through the point of vision.

Notwithstanding the facility of construction, the stereographic projection is not much used in map-making. It is sometimes used for maps of the hemispheres in atlases, and for star charts.

External Perspective Projection.-We now come to the general case in which the point of vision has any position outside the sphere. Let abcd (fig. ro) be the great circle section of the sphere by a plane passing through c, the

central point of the portion of surface to be represented, and V the point of vision. Let pj perpendicular to Vc V Flo. Io.

be the plane of representation, join mV cutting pj inf, thenf is the projection of any point m in the circle abc, and ef is the representation of cm. Let the wangle com=u, Ve=k, V0=h, ef=p; then, since ef: eV== mg: gV, we have p =k sin u/(h-l-cos u), which gives the law connecting a spherical distance u with its rectilinear representation p. The relative scale at any point in this system of projection is given by a =dp/du, <r'=p/sin u,

o =k(I +h cos u)/(h, -l-cos u)2;a'=k/(h-1-cos u), the former applying to measurements made in a direction which passes through the centre of the map, the latter to the transverse direction. The product aa' gives the exaggeration of areas. With respect to the alteration of angles we have E = (h+ cos u)/(1 -I-k cos u), and the greatest alteration of angle is h- I 2?

- sm -h+Itan 2).

This vanishes when h= I, that is if the projection be stereogra hic; or for u=0, that is at the centre of the map. At a distance of) 90° from the centre, the greatest alteration is 90°-2 cot-lx/h. (See Phil. Mag. 1862.)

Clarke's Projection.-The constants h and k can be determined, so that the total misrepresentation, viz.: M =j, '3{(¢r-1)2+(u'-1)2} Sin uae,

shall be a minimum, B being the greatest value of u, or the spherical radius of the map. On substituting the expressions] for a and a the integration is effected without difficulty. Put, )=(I -cost?)/(h+ cos /S); v= (h-I)), H =z/-(h-|-1) log, ,()+1), H' =}(2 -v+§ v2)/(h+I). Then the value of M is

M =4 sin* é /3-l-2kH+k'H'.

When this is a minimum,

dM/dh=o; dlVI/dk=o

kH'-1-H =o; 2dH/dh-l-kdh. H'/dh =o. Therefore M =4 sin' iii?-H2/Hl, and h must be determined so as tv make H2:H' a maximum. In any particular case this maximum can only be ascertained by trial, that is to say, log H2-log H' must be calculated for certain equidistant values of h, and then the