Page "Christiaan Huygens" Paragraph 25

Such springs are essential in modern watches with a detached lever escapement because they can be adjusted for isochronism.

Based on this fact, many writers in the past and even the present have given Huygens the credit for inventing the modern form of spiral balance spring in 1675 rather than Hooke's earlier straight spring of 1665 because they considered that the spiral form automatically conferred the property of isochronism on the oscillating balance.

However, this assumption is incorrect, as all watches in the time of Huygens and Hooke employed the very un-detached verge escapement, the action of which destroys the isochronal properties of any form of balance spring, spiral or otherwise.