diff --git a/README.md b/README.md
index 67e892a..f7c2cf0 100644
--- a/README.md
+++ b/README.md
@@ -105,8 +105,19 @@ Most metals however aren't purely reflective, they fuzz the reflection a little
 
 <img src="https://raytracing.github.io/images/img-1.14-metal-fuzz.png" width="480"/>
 
-### Chapter 9: Glass
+### Chapter 9: Glass and Slowness
 
 In this chapter glass and other dielectric materials have been implemented using Snell's law which describes refraction. The first attempt yields the following rendering:
 
-<img src="https://raytracing.github.io/images/img-1.16-glass-always-refract.png" width="480"/>
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+<img src="https://raytracing.github.io/images/img-1.16-glass-always-refract.png" width="480"/>
+
+In the later part I implemented Schlick's Approximation which approximates the reflectivity of dielectric objects depending on the angle.
+
+Here I got some strange result (actually I already noticed that the previous render had a strange cyan sheen on the upper part of the ball, but dismissed it as a result of inaccuracies).
+
+I oddly enough got this wrong result, when trying to render the hollow ball:
+<img src="https://imgur.com/a/ylcvwWi" width="480"/>
+
+It took me some time to find the error and believe it or not, it was the 3% inaccuracy caused by the fast inverse square algorithm! I had to remove it and replace it with a more simple and accurate but slower calculation. The slow down is small but visible! Now I get:
+
+<img src="https://raytracing.github.io/images/img-1.18-glass-hollow.png" width="480"/>
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