His genius is most visible in his _autos sacramentales_, a dramatic form peculiar to Spain. The word _auto_ is first applied to any and every play; then, the meaning becoming narrower, an _auto_ is a religious play, resembling the mediæval Mysteries (Gil Vicente's _Auto de San Martinho_ is probably the earliest piece of this type). Finally, a far more special sense is developed, and an _auto sacramental_ comes to mean a dramatised exposition of the Mystery of the Blessed Eucharist, to be played in the open on Corpus Christi Day. The Dutch traveller, Frans van Aarssens van Sommelsdijk, has left an account of the spectacle as he saw it when Calderón was in his prime. Borne in procession through the city, the Host was followed by sovereigns, courtiers, and the multitude, with artificial giants and pasteboard monsters—_tarascas_—at their head. Fifers, bandsmen, dancers of decorous measures accompanied the train to the cathedral. In the afternoon the assembly met in the public square, and the _auto_ was played before the King, who sat beneath a canopy, the richer public, which lined the balconies, and the general, which filled the road. Even for an educated Protestant nothing is easier than to confound an _auto sacramental_ with a _comedia devota_ or a _comedia de santos_: thus Bouterwek, in his _History_, and Longfellow, in his _Outre-Mer_, have mistaken the _Devoción de la Cruz_ for an _auto_. The distinction is radical. The true _auto_ has no secondary interest, has no mundane personages: its one subject is the Eucharistic Mystery exposed by allegorical characters. Denis Florence M'Carthy's version of _Los Encantos de la Culpa_ (The Sorceries of Sin) enables English readers to judge the _genre_ for themselves:— Sin. "_... Smell, come here, and with thy sense Test this bread, this substance,—tell me Is it bread or flesh?_ The Smell. _Its smell Is the smell of bread._ Sin. _Taste, enter; Try it thou._ The Taste. _Its taste Is plainly that of bread._ Sin. _Touch, come; why tremble? Say what's this thou touchest._ The Touch. _Bread._ Sin. _Sight, declare what thou discernest In this object._ The Sight. _Bread alone._ Sin. _Hearing, thou, too, break in pieces This material, which, as flesh, Faith proclaims, and penance preacheth; Let the fraction by its noise Of their error undeceive them: Say, is it so?_ The Hearing. _Ungrateful Sin, Though the noise in truth resembles That of bread when broken, yet Faith and Penance teach us better. It is flesh, and what they call it I believe: that Faith asserteth Aught, is proof enough thereof._ The Understanding. _This one reason brings contentment Unto me._ Penance.