The solar microscope, which more properly could be called a projection microscope, is a device that projects onto a screen real, greatly enlarged images of objects, the details of which are too small to be observed by the naked eye.
The name solar is due to the fact that in the first instruments, in order to have a sharp image, the object was intensely illuminated with sunlight using a heliostat; but the denomination remained unvaried even when other light sources, such as the arc lamp, were used.
The German physician and naturalist Johann Lieberkühn (1719-1769) can be considered the inventor of the solar microscope. He used it, starting from 1740, in the study of very small animals, illuminating them with sunlight with a device that bears his name, but the model that became widespread was the one designed and built by the London optician John Cuff (c. 1708-1772) who was the first to use it with a heliostat.
The parts of the exemplar on display, which is the one that was most widespread in the 19th century, are:
- the brass tube consisting of two pieces, one a truncated cone (diameters 9 and 6 cm; length 14.5 cm) and the other a cylinder which slides, with an extension of 11 cm, inside the narrower part of the former;
- the shelf for objects on which the support for the slides moves; it is burnished brass, at the centre it has a circular aperture and lies with four pins with spring spacers on another burnished brass plate attached to the narrower end of the tube;
- the objective, composed of a system of converging lenses of short focal length held by a staff which, by means of a rack screw, allows it to move towards or away from the slide.
At the wider end, the microscope tube is threaded so that it can be screwed onto the heliostat and it has a converging lens which, by completely covering the aperture, captures the sunlight reflected by the heliostat mirror, making it converge onto one lens placed at the end of the tube which, in turn, makes it converge on the object.
The distance between the two lenses and the convergence of the sunlight can be adjusted by manually moving the cylindrical part of the tube up and down inside the other part and then making fine adjustments by means of a rack screw.
To have the object in sharp focus and sufficiently enlarged on the screen so that its details can be observed, the position of the objective is adjusted by turning the appropriate rack screw so that the object falls beyond the focus at a distance from it of not more than the focal length.
Boutan - D'Almeida (1867), T. II, p. 363
Daguin (1879), T. IV, p. 150
Roiti (1888), Vol. II, p. 83
Turner (1987), p. 120