Why I must see the blue discharge arc “the lightning” with my own eyes and measure near the edge of the glass?

Sparklike Handheld™ measures the colour of the arc and from that specifies the gas content. If you cannot see the arc, Sparklike Handheld™ does not either.

Typically, the discharge happens easier once the system gets grounded in
the edge area. Therefore, measuring near the edge is important.