The island was seen as straight. And yet the isle of Wight is 22 miles in length total.

It didn't look like this picture below. If the earth were really curved, you would get this picture. A big obvious brainer here

1. That is not a picture, it's a drawing.

2. The Isle of Wight is 22 miles long. That is 35.2 km.

The curvature of the Earth in 35.2 km is 97m. Simple calculation, Pithagorean theorem: (Ro+h)^2=Ro^2 + L^2 ; where Ro is Earth Radius, L is the length of the island, and h is the height level difference between the two ends of the island. Using Ro=6371 km and L=35.2km, you get h=97m.

This seems significant at first, until you compare L with h.

L/h = 35.2km / 97m = 363.

That means, the length of the island is 363 times greater than the height difference between its ends.

Meaning, even if you took a picture with perfect parallel projection (which isn't possible), the height level difference at the ends of the island would be 1/363 of the length of the island. If the island on the picture would be 363 pixels wide, you'd see a one pixel drop (not on both sides, only on one, or half a pixel of drop on both sides). This isn't something you'd notice with a telescope, you'd think you're seeing a straight line. As far as your eyes can tell, you'd be looking at the first image, on a globe Earth, just the same.

Seriously, you'd only need to think for just a little bit, and you'd realize why seeing curvature on that island is impossible with an optical telescope.