1. 35. Poggendorff Illusion
The Poggendorf illusion in all its glory. You almost need a straightedge to follow the lines.
Focus on the black line and follow it downwards. Is it continuous with the blue line or the red line?
The illusion does not seem to persist through a rotational change. Below, the entire image has been rotated by 31 degrees. The effect is gone.
Interestingly, the flag of the United Kingdom is designed with shifted diagonal lines.
This illusion was discovered in 1860 by physicist and scholar JC Poggendorff. While pondering another illusion, Poggendorff noticed and described this illusion resulting from the apparent misalignment of a diagonal line; an illusion which today bears his name.
This was done perhaps to compensate (the bottom-left to top-right red line) and to “over-compensate” (the other oblique red line) for this effect. The green line is drawn to bring this out.

2. 36. Color Contrast Illusion
In the image to the right, there is a red overlay over half the face except for the eye. Do the two eyes have the same color?
Obviously, the two eyes have the same color. The redish tint next to the eye shifts the neutral gray of the eye to the opponent colour, which for red is blue-green.
Most people see the right eye as grey and the left eye with a bluish tint.

3. 37. Fraser’s Spiral Illusion
One of the reasons for the illusion is that the checkerboard background pattern appears to trace out spirals. When it is removed, the illusion is lessened somewhat. But the components next to the bands still follow a spiral path.
The image above is named Fraser’s Spiral. James Fraser developed it in The illusion is also known as the false spiral or the twisted cord illusion. Click to see why.
The overlapping black arc segments do not form a spiral but concentric circles.

4. 38. Stroboscopic Motion Illusion
The 2 blue dots are changing position. Are they moving horizontally or vertically?
In reality, there is no motion. The 4 dots are staying put but their visibility is switched. The motion occurs in our brain.
If you see them moving vertically, we cover up the bottom 2 dots. What do you see now?
If you see them moving horizontally, we cover up the left 2 dots. What do you see now?

5. 39. Bird Illusion
When you click you will see a bird moving. The bird will stop and merge with the background. You might see part of it but most likely it will fade away until vanishing completely.

6. Sources
35. Poggendorff -
36. Color Contrast -
37. Fraser’s Spiral -
38. Stroboscopic -
39. Bird -