The widely known Hermann Grid illusion (Hermann 1870). In nearly any optical illusion collection.

Dark patches appear in the street crossings, except the ones which you are directly looking at.

The inverse:

Sources:

Hermann L (1870) Eine Erscheinung simultanen Contrastes. Pflügers Archiv für die gesamte Physiologie 3:13–15

Aspirin went on sale as the first pharmaceutical drug in 1899, after Felix Hoffman, a German chemist at the drug company Bayer, successfully modified Salicylic Acid, a compound found in willow bark to produce Aspirin.

Cocaine was the first local anesthetic; being used as such from about 1884 onwards.

Cocaine works in a totally different way from narcotics such as morphine or heroin. Heroin works on receptor sites in the brain which are stimulated by the drug to produce pain-relieving and mood-enhancing chemicals. Cocaine on the other hand works by stimulating the central nervous system, and like alcohol, is processed through the liver.

‘Crack’ is the street name given to cocaine that has been processed from cocaine hydrochloride to a free base for smoking. Rather than requiring the more volatile method of processing cocaine using ether, crack cocaine is processed with ammonia or sodium bicarbonate (baking soda) and water and heated to remove the hydrochloride, thus producing a form of cocaine that can be smoked. The term ‘crack’ refers to the crackling sound heard when the mixture is smoked (heated), presumably from the sodium bicarbonate. ‘Crack Cocaine’ is still cocaine. It is simply a different chemical process applied to cocaine powder that allows cocaine to be smokeable. This means that the ‘high’ from Crack Cocaine is much stronger and more immediate (taking about 8 seconds to reach the brain); and also shorter lived then from the powder. =continued=

Why is the day divided in 24 hours? Why not 7 or 17?

The History of Analog Clocks provides an explanation of the origin of the 24-hour day. The system dates back 4,000 years, to ancient Egypt and Babylon.

Egyptian astronomers divided the night hours into 12 distinct sets of star patterns, which emerged over the horizon at roughly hour-long intervals. They divided the day into 12 hours to match, and because they lived near the equator, the days and nights were of roughly equal length throughout the year.

Throughout the rest of the world, the 12 “hours” of the day and night were flexible depending upon the season. It wasn’t until the 13th century A.D. that an Arab scientist named Abul-Hassan suggested the idea of clearly defined hours.

It’s also worth noting that the number 12 is special in many cultures. For example, Egyptians counted in base 12. Dividing the day into two sets of 12 was a simple and elegant way of condensing the annual calendar into a day.