camera system object centric

I have this function in webgl for a camera system:

function handleMouseMove(event) {
            if (!mouseDown) {
            var newX = event.clientX;
            var newY = event.clientY;

            var deltaX = newX - lastMouseX
            var newRotationMatrix = mat4.create();
            mat4.rotate(newRotationMatrix, degToRad(deltaX / 10), [0, 1, 0]);

            var deltaY = newY - lastMouseY;
            mat4.rotate(newRotationMatrix, degToRad(deltaY / 10), [1, 0, 0]);

            mat4.multiply(newRotationMatrix, RotationMatrix, RotationMatrix);

            lastMouseX = newX
            lastMouseY = newY;

the problem is that this camera system is not object centered.
I wish, when rotate , rotates the object around its axis and for the traslation go ahead and back.
I already done for traslation and mouse weel, but i don’t know how do for the rotation of the camera around object.
How i can find the object axis? i have a bounding sphere, and his center.