Jan Engelman certainly assigned himself an unenviable task when he wrote Het regt gebruik der natuurbschouwingen... in 1747. This work was an attempt to categorize snow crystals by their different forms. Didn't poor Engelman know that no two snowflakes are alike? He made drawings of over 400 forms of crystals, and encouraged his readers to provide him with drawings of any he missed. He hoped to provide reader generated addendums to his book, but this never happened. Engelman looked to correlate various forms of snow crystals with such factors as different temperatures when formed. Item 37. €1,750 (US $2,220).

For those who collect the drug culture, item 1 is an early tract. From 1814, it is Considerations Physiologiques et Medicales Sur L'Opium by Frederick Amiel. This is an everything you ever wanted to know about opium book, including the symptoms of taking it, whether it is a medicine or a poison, and how to choose opium of good quality. €600 (US $761).

There aren't a lot of early books on swimming, but here is one: L'Art de Nager....by Melchisedech Thevenot. Now that's an uncommon name. Thevenot was more noted for writing books on early voyages, but this one proved to be a popular exception. Published in 1696, it was a standard teaching guide in the 18th century. Item 118. €5,000 (US $6,342).

Item 150 is one of those tests we have all taken, or at least all of us of the male gender. It is Dr. Shinobu Ishihara's Tests for colour-blindedness. This is a 1975 set of 38 plates used to determine whether a person can see colors. Dr. Ishihara devised a system where numbers, formed from a pattern of circles, are hidden within other circles of a different color. If a person can distinguish between the colors, the numbers will be visible. If not, the page will just look like a bunch of random circles, indistinguishable from one another. You can check the image on this page to test whether you can see the green "16" among the red and yellow circles. However, this should not be considered a definitive test as the reproduction of these colors on your computer may not be accurate. You should see an optometrist if you have any concerns. €50 (US $64).

Frits Knuf Antiquarian Books may be found online at www.fritsknuf.com, phone number +33(0)254-722656.