Nested transparent spheres - Can't visible the core

Hi all,

I'm trying to create layered spheres around a cube. Normally it should be transparent for each layer, but as the layers increase, the cube is not visible. How do I ensure the transparency of each layer equally? I just want the layers to be clearly visible but at the same time, I want to increase the visibility of the centre.

The button you see in the picture adds a layer each time and the central cube loses its visibility a little more each time.

Thanks.