This idea is not difficult to grasp when we think about the gaze of ourselves in the mirror or the gaze of other people, but it can be applied to objects, which might also be said to 'send back our sight'

你对句子结构理解错误。but...并不是插入语，而并列连词并列二个分句。which引导非限制性定语从句，先行词为objects。语法上不允许这个定语从句修饰but前的另一个分句的名词。it指代this idea。但this idea与to send back our sight之间没有逻辑主谓关系，所以可以排除which的先行词为it。

which 指代 objects。所以：

which might also be said to 'send back our sight'

= objects might also be said to 'send back our sight'

= it might also be said that objects will send back our sight.

整个句子的意思是，this idea不但适用于人，也适用于物体。适用于人很好理解，适用于物体就较难理解。我们和别人或镜子中的自己对视时，会感到我们看别人，别人也在看我们。同理，我们看物体时，也可以了解物体也在看着我们。这就是作者要表达的意思，翻译时你自己组织语言。