Compatible with all existing Welch Allyn 3.5 V power sources. Look for the traditional "flat-top" design you've come to trust. The patented Welch Allyn Coaxial Vision System facilitates ophthalmoscopy by eanabling easier entry into the eye, a larger field of view, and reduced glare compared to standard ophthalmoscopes. Also known as 3.5V Autostep.