The present volume is chiefly intended for those of my young friends who have read, and been interested in, the Fairyland of Science. It travels over a wide field, pointing out a few of the marvellous facts which can be studied and enjoyed by the help of optical instruments. It will be seen at a glance that any one of the subjects dealt with might be made the study of a lifetime, and that the little information given in each lecture is only enough to make the reader long for more.<br />In these days, when moderate-priced instruments and good books and lectures are so easily accessible, I hope some eager minds may be thus led to take up one of the branches of science opened out to us by magic glasses; while those who go no further will at least understand something of the hitherto unseen world which is now being studied by their help.