The megalethoscope is a larger version (mega-) of the alethoscope, (Italian: alethoscopio, from the Greek “true”, “exact” and “vision”) which it largely superseded, and both are instruments for viewing single photographs with a lens to enlarge and to create some illusion of three-dimensionality. They were used to view photographic albumen prints that were coloured, perforated and mounted on a curved frame. Night effects were achieved when viewing pictures in transmitted light from a fitted oil or kerosine lamp and a daytime version of the same scene was seen when lit by the reflected light from two side mirrors. They are sophisticated versions of the peep show, and were designed by Carlo Ponti of Venice before 1862. Lke the similar graphoscope which descends from the eighteenth century zograscope predating photography, these devices were, and are, often confused with the stereoscope which was of a different design and effect. Improvements to the megalethoscope over the alethoscope, mainly the addition of a compound lens, are detailed in The Practical Mechanic's Journal of 1867.

Optician, photographer and publisher of views for the tourist and art-connoisseur markets, Carlo Ponti invented the alethoscope in 1860. He presented the device to the Société française de photographie in 1861, then in April, to the Istituto di Scienze, Lettere ed Arti in Venice, earning an honourable mention there in May. He obtained a patent in January 1862 and commenced marketing it and photographs to be viewed using the instrument. His invention was awarded Grand Prix at the International Exhibition in London in 1862. The megalethoscope was produced for him by cabinetmaker Demetrio Puppolin, whose name is inscribed on different models. It was a substantial status symbol, an often elaborate item of furniture that only the well-to-do could afford; some are highly decorated with pearl inlay and marquetry, and they often held collections of photographs in a cabinet beneath.

The megalethoscope was for the viewing of single, usually 25 x 34 cm., albumen prints larger than those for the alethoscope. Ponti produced and published his imagery mostly for the tourist market in the waning era of the Grand Tour, but also for connoisseurs of art and architecture, in large quantities and to an international clientele through outlets in Europe, England and America.

The megalethoscope and the alethoscope are capable of a certain illusion of relief. Photographic views are seen enlarged through a wide, thick magnifying glass that creates an illusion of the subjects' plasticity, perspective depth and modelling. The instrument's arrangement minimises surrounding indicators of depth that would let us know this is a flat picture, and also because the image is magnified to nearer the scale of the real scene the picture is depicting. As the light coming from the lens to the eyes is collimated, it confounds accommodation; the image appears suspended at an indeterminable range. The broad, thick lens could also enhance depth perception by creating binocular stereopsis, because each eye views the image through a different part of the magnifying glass; chromatic aberrations at the edges of the lens may contribute to chromostereopsis; and depth clues in the image, which were usually architectural interiors or exteriors in perspective, help to create the illusion.

The Practical Mechanic's Journal of 1867 noted minor improvements over the alethoscope that were made to later models of the megalethoscope, but the relative size of the two otherwise similar instruments is their distinguishing difference;

1. The two lenses [one in front of the other] which may be removed through the small door for the purpose of cleaning them, instead of one fixed lens;

2. the substitution of several diaphragms, opening on a hinge...so as to inclose [sic] large and medium sized photographic views and cartes de visite, for the single diaphragm used in the Alethoscope;

3. the method of fixing ordinary portraits and small photographic views.