The 35RC has a split-image rangefinder - as you look through the viewfinder, there is a rectangular field in the middle of the frame that is slightly yellow. As you focus the camera, you will see that there are actually two versions of the image within that yellow field, and focusing the camera makes one image move with respect to the other. When the two images are lined up exactly, the camera is focused.
Rangefinder focusing works best when there are lines in the image that can be used to focus - it's much easier to align two images of an edge than it is to align two images of a textureless field.
There are two problems that you may encounter with the 35RC. Either my eyes are starting to fail (well, I have had cataract surgery on one eye - getting old is hell) or else one of the images is starting to fade, but it seems to me that its harder to see those two images than it used to be. Of course, I've had my 35RC for 31 years at this point, which is more than half of my life, so either explanation may be correct.
The other problem is that the 35RC used a mercury battery that is nearly impossible to replace. When mercury batteries were being phased out, there was an article in Pop Photo talking about the options that made a specific point of mentioning that some of the available options wouldn't work for the 35RC. Don't recall all the details, but the article specifically cautioned that using a battery with a slightly higher voltage was potentially dangerous in the 35RC because the wiring in the meter circuit couldn't handle larger currents that would flow with a higher battery voltage.
I've thoroughly enjoyed my 35RC - my job involved a lot of travel, and I bought the 35RC specifically to live in my briefcase so that I would always have a camera. It probably has over a million air miles at this point!