A systematic method for the optical design and analysis of a multipass cell (MPC) with two spherical mirrors is presented. By varying the parameters, including the distance between the mirrors, the position of the incident point, and the incident angles, a variety of dense patterns can be formed on the mirrors. Three MPCs with exemplary patterns of seven circles, rotating triangles, and petals are designed and built. We compare and analyze the parameters of each MPC with respect to the volume, the optical path length (OPL), the number of reflections, and the stability of the final spot position. This new type of MPC exhibits the superior characteristics of compactness, a long effective OPL, and affordable cost, which make it very suitable for applications to trace gas sensing.