Lowell, Venus, and aperture

I was reading the current issue of S&T how Lowell had discovered the spokes of Venus. I stumbled across the info that he found that Venus was "too bright and badly shattered by air currents" in the new 24" Clark refractor. So he stopped down the aperture (using the iris). He found the spokes was best seen with aperture between 1.6" to 3"!
Wow, I gave up too easily on Venus, thinking these features would need excellent seeing and large aperture :-(. I now wonder if Lowell were to continue to use full aperture, he may never have discovered the spokes!

I've read about other instance at another observatory where the aperture must be reduced (in this case to 4") to get a good view of Mars. But I've also read here and on SAA that full aperture can see more in *bad* seeing! So these two observation approaches seems contradictory. Also, if reduced aperture isn't helpful, why do these observatory class telescopes have built-in iris?

Can anyone/everyone shine some more light into aperture ;-)?

Thanks,
Ron B[ee]

One reason those old observatory refractors have a built in iris is for reduction of chromatic aberration. Remember, those are achromats and very large ones at that. For bright targets that didn't need the full aperture the iris let the observer stop down the instrument to effectively longer focal lengths and greatly reduce the chromatic abberation.