I will soon get,along (with some other stuff, a smallish brass lens, marked Darlot No. 2

I don't know much about these old brass lenses except that the old Petzval formula is now trendy for the "swirly Bokeh". . . .what the next 50-100 years of lens design tried to eliminate.

Is there an online reference that might give an introductory overview of the various models and configurations of the lenses from Darlot?