A Ronchi screen make it easy to understand for the both the expert and beginner to what is going on ie straight bands is good, not straight not so good. The double pass test is the use of the optical flat but it doesn't have to use a Ronchi screen to examine the returned image. You can use a knife edge.
I don't think that the Ronchi test was invented when the Clark's first started to use the test. What double pass is doing is making an artificial star. So if the light source is a pin hole and you use an eyepiece your going to see an image of that artificial star so you can do a star test.
I think the drawing in the article is not what they were doing and not just using an eyepiece but also a knife edge so they were looking for a null. If they did not see one they were seeing the zone errors and then could polish the lens to remove them.
Edited by DAVIDG, 27 February 2018 - 09:51 AM.