Letter, from [James] Gregorie to [John Collins], dated at St Andrews Eclipse of last March was not seen in Scotland because of a storm. Discusses how [Robert] Anderson misunderstood some of Gregorie's prior work in Stereometrical Propositions and develops the correct demonstration. Notes that despite [Isaac] Barrow being "a most excellent and learned geometer" he has made some errors in his demonstration published in Dioptrick, corrects the errors. Last paragraph underlined in red. Endorsed 'No. 15: p 95' in reference to 'Commercium Epistolicum' 1722 edition, page 95, paragraph XV Headed 'About Andersons Booke. Some Errors in Dr Barrows Dioptricks' in John Collins' hand. Numbered 7 likely by Collins on the first page. 20 April 1670