This adage for my students was obviously inspired by "He who steals my purse steals trash" but the lens borrower may be in for more serious trouble than the purse snatcher. This comes about because the eyepiece fit and the objective threads are standardized to a couple of standards so that the mechanical interchange of components between different optical microscopes is easily accomplished, while the incompatibility of the optical components is not usually apparent. This incompatibility can range from easily recognized blurring of the image to only slight loss of resolution which leads to inferior work or much wasted time before the problem is recognized.