A novel construction for electrically illuminated medical diagnostic instruments, particularly otoscopes and ophthalmoscopes. The invention fills a need for instruments that are relatively low cost and yet of good quality, the lower cost being achieved in part by the materials used and in part by the ease with which the components can be assembled. In addition to being economical, the instruments disclosed are easy to use and maintain. They are lightweight and compact, and for ease of maintenance are provided with an easily removable light source unit in which the lamp and batteries are mounted. This greatly facilitates changing these expendable items when such becomes necessary.