How can I tell if a lens is a Petzval?

[PhotoPete]

I just aquired a number of old lenses, including a Darlot. How can I tell if this or any of the others are Petzvals, RR's or something else. I recall seeing a post about counting the number and sizes of the reflections from a small lilght source, but can't seem to locate it now.

Any help or hints is appreciated.

 

[jimgalli]

Take a small diode penlight and hold it about 18" away. With aperture closed all the way down, or with cells indiviually removed from the barrel, shine the lamp in and you will see some reflections of your source. A Petzval has bright dim bright in front which is what you get from a cemented doublet. The rear will have bright bright bright bright which is what you would get from 2 glasses with an air space between. An RR will look just like the front of the Petzval at both ends. A Dagor type will have bright dim dim bright (sometimes out of order and or different sizes) which is what you get with 3 glasses all cemented. A Protar VII will have bright dim dim dim bright.