Dielectric - Hollow sphere slightly odd Reflection/Refraction

[Bashmelek]

In the Hollow Sphere, for the dielectric material, it is reflecting some of the middle sphere and in general is refracting differently then the example picture. I've tried reviewing my code, but so far nothing sticks out to me. Plus, I am using C++ and basically just typing what I see, so even as I compare nothing looks wrong--if even different at all--anywhere. Does anyone have any idea what could be the issue?

My codebase is here: https://github.com/Bashmelek/RayTracingIn1

[Bashmelek]

Found it! Issue was that the hollow sphere is made of two spheres, with opposing normals:

world.add(make_shared<sphere>(point3(-1.0, 0.0, -1.0), 0.5, material_left));
world.add(make_shared<sphere>(point3(-1.0, 0.0, -1.0), -0.4, material_left));

I was only using the second one.