diff --git a/chapters/lightfields-04-properties-spectral.qmd b/chapters/lightfields-04-properties-spectral.qmd
index fbb268f..863d10d 100644
--- a/chapters/lightfields-04-properties-spectral.qmd
+++ b/chapters/lightfields-04-properties-spectral.qmd
@@ -25,7 +25,7 @@ https://cie.co.at/datatable/components-relative-spectral-distribution-daylight
 
 Isaac Newton discovered that light, say from the sun, could be separated into different wavelengths. @fig-newton-prism shows Newton's dorm room in which he closed his window with a shutter, leaving only a small hole for the rays of light from the sun to enter.  In the middle of the sketch we see a lens and a prism, which separated the rays of light according to their wavelength into an image on his wall. The diagram indicates several critical experiments that Newton performed to show that the separation was fundamental and not destructive. I describe these in more detail in [Foundations of Vision](https://wandell.github.io/FOV-1995/chapter-4-wavelength-encoding.html).
 
-![Newton's diagram showing how he used a prism to separate light into 'rays of differing refrangibility'.](images/lightfields/04-spectral/26_newton-prism-experiment-lines.png){#fig-newton-prism width="70%"}
+![Newton's diagram showing how he used a prism to separate light into 'rays of differing refrangibility' [@newton-prism-diagram].](images/lightfields/04-spectral/26_newton-prism-experiment-lines.png){#fig-newton-prism width="70%"}
 
 ::: {#nte-newton .callout-note title="Newton: The greatest physicist?" collapse="true"}
 ![Portrait of Sir Isaac Newton, 1689, by [Godfrey Kneller](https://commons.wikimedia.org/wiki/File:Portrait_of_Sir_Isaac_Newton,_1689.jpg). Public domain, via Wikimedia Commons.](images/lightfields/04-spectral/newton_kneller_1689.jpg){#fig-newton-portrait width=40% fig-alt="Oil portrait of Isaac Newton at about age 46, with long hair, in dark clothing."}