Family and early life: His parents were Thomas Callan from Drung, Co. Cavan and Bridget Farrelly from Finternagh, Bailieborough, Co. Cavan. They married on 29 November 1844 in Killenkere, Co. Cavan. Their witnesses were Henry McGinn, Rose Brady and Sarah Farrelly.
Their son Peter was born on 10 October 1845. He was either born in Finternagh or Drung, Co. Cavan or Delaware, Newark, New Jersey, birth or baptismal records cannot be found anywhere to prove or disprove this. His mother Bridget died when he was young and his father married Maria Smith on 21 February 1852 in the Parish of Collon. Their witnesses were James Smith and Susan Kerrigan.
On 22 April 1852, Thomas age 30, his new wife Maria, age 20 and Peter age 7 arrived in New York on board the Saratoga having departed from Liverpool.
If Thomas and Peter were in America before this, you may wonder how did Thomas meet Maria? The answer lies in the family tree. Thomas’s sister Margaret Callan (1812-1900) was married to Patrick McCabe, their daughter Mary McCabe (1835-1883) married Hugh Smith from Drung, Co. Cavan (1840-1907), Maria was Hugh’s sister. Hugh & Mary also emigrated to America.
In 1860, Peter was living with his father, a grocer, Maria and James (his half brother), a student in Newark, Essex, New Jersey, this record stated that he was born in Ireland 
He graduated in medicine from the University Medical College of New York City in 1867. In 1870, he was living with his father, Maria and James and he stated that he was a physician and born in New Jersey. On July 16, 1871 his father Thomas passed away.
Peter married Angele I. Beckwith White (1852-1933) in Louisville, Jefferson, Kentucky, on July 20, 1876, when he was 30 years old. The celebrant was the Roman Catholic Bishop of Louisville, William George McCloskey and their witnesses were John & Martha White. They had six children Lewis, Harold, Gerard, Angele, Raymond and Kenneth.
A brief outline of his career:
1867: He graduated in medicine from University Medical College of New York City.
1867-68: House surgeon Charity Hospital, New York.
1868-72: Asst. Surgeon U. S. Navy, during this time he travelled to Europe.
1874: Attending ophthalmic surgeon, Central Dispensary, New York City.
1877: Consulting surgeon, N. Y. Eye and Ear Infirmary; ophthalmic surgeon, St. Vincent's and Columbus Hospitals.
1888: Elected a Fellow of the New York Academy of Medicine.
He was also a member of County Medical Society, State Medical Society, America Academy of Ophthalmic Society, (being president of this society in 1916 and 1917), Mutual Aid and American Medical Associations. His practice was at 452 Fifth Ave., N. Y. City.
When he was applying for his passport on Jan 1921, he stated he was living at 156, Park Avenue, Yonkers. Then he signed an affidavit stating that he had no living relative except his wife and children and that he was informed that his baptismal or birth certificate could not be located, but that he was born in Newark in 1845. The purpose of this passport was to allow him travel abroad for health purposes to Haiti for six months. Previously he travelled and lived abroad for short periods from January 1916 to March 1916 in San Domingo and from January 1918 to March 1918 in Porto Rico all for health purposes.
He died on October 4, 1932 in his 87th year. His wife Angele I. White Callan, two sons Harold and Raymond Callan and a daughter Mrs Angele I. Reynolds survived him. His sons Kenneth in 1911, Gerard in 1918 and Lewis in 1920 predeceased him.
Some of the testimonies to his career:
The New York Times. Oct.18, 1911
Boys’ Sight May Be Saved - Remarkable Operation Promises to Repair Damage of Dynamite Caps.
Walter McIntyre and Nathan Lake, two Yonkers boys, pocketed some dynamite caps they found in an excavation where blasting was being carried on in the Park Hill suburb of Yonkers, several weeks ago, and carried them to the woods to pry off the copper setting of the caps. There was a terrific explosion, in which the eyes of both boys were riddled with bits of copper. After treatment their sight was despaired of.
Several days ago, however, Drs. George B. Stanwix and Peter A. Callan of Yonkers, advised the removal of the boys to the New York Eye and Ear Infirmary, where an operation was performed by the two physicians. Afterwards both boys were able to see glimmers of daylight. It was said last night that both would probably recover their sight. Dr. George B. Stanwix was elated last night over the success of the operations, which were among the rarest in several years at the Eye and Ear Infirmary. “When the boys first came under my care” he said last night, “their bodies from their waists to their hair, were filled with particles of dirt, bits of copper and of bark. It seemed impossible to save their lives, let alone their sight. “Little by little through the summer we have got these particles out of their skin. From each eye of each boy, I removed between 150 and 200 bits of copper. Each piece was removed by cutting with a knife. The final effort to restore their sight was made at New York Eye and Ear Infirmary on Monday by Dr. Peter A. Callan and myself. “The operation was for the removal of traumatic cataract and is an extremely delicate one. It is performed with the knife, an incision being made in the upper part of the eye and the film, which covers the eyeball drained off. It was successful in each case, and I expect the boys shortly to have the full use of their eyes again”.
Wise State Journal, Madison: July 2 1921. Mme. Curie may lose eyesight
Reports that Mme. Curie, co-discoverer who with her husband conducted pioneering research on radioactivity, is threatened with blindness, are confirmed by Dr. Peter A. Callan, oculist, who says that few days before Mme. Curie left last Saturday for France on the steamship Olympic, she called to his New York office.
The Ophthalmoscope in the Lifetime of Hermann von Helmholtz: In the attempt to automate the introduction of a wide range of sequential convex and concave lenses in a conventionally shaped head, perhaps the Roth and Callan ophthalmoscopes are the most ingenious. Although both ophthalmologists published details of their instruments in 1864, Peter Callan’s visit to Berlin most likely allowed him to observe August Roth’s design and then imitate it. Only in detail do they differ.
The Ophthalmoscope: Peter A. Callan in America in 1894 combined the model of an ophthalmoscope by August Roth (1858-1930) with the last by Edward Greenly Loring (1878) and published this in 1894.
Dr. Callan was a contemporary of Dr. Roth and his instrument is similar in construction. The main interest of this non-luminous ophthalmoscope is the internal mechanism, which is unseen. The operator rotates the lower wheel to change the lenses which range in power from 1D to 24D in one Dioptre steps in both the plus and minus. Additionally a plus 0.50D lens can be introduced directly behind the aperture of the mirror by pulling on a vertical column and holding it in position. The ophthalmoscope has a single Loring type concave mirror, which can be tilted left and right.
 "United States Census, 1860," database with images, FamilySearch (https://familysearch.org/ark:/61903/3:1:33SQ-GBS6-93Q4?cc=1473181&wc=7Q3... : 24 March 2017), New Jersey > Essex > 7th Ward Newark City > image 76 of 207; from "1860 U.S. Federal Census - Population," database, Fold3.com (http://www.fold3.com : n.d.); citing NARA microfilm publication M653 (Washington, D.C.: National Archives and Records Administration, n.d.).
 "United States Census, 1870," database with images, FamilySearch (https://familysearch.org/ark:/61903/3:1:S3HT-6LZS-V5T?cc=1438024&wc=92KF... : 13 June 2019), New Jersey > Essex > Newark, ward 07 > image 131 of 302; citing NARA microfilm publication M593 (Washington, D.C.: National Archives and Records Administration, n.d.).
 The Boston Herald, 5 October 1932.
 C. Richard Keeler, ‘The Ophthalmoscope in the Lifetime of Hermann von Helmholtz‘ in Arch Opthalmol, Vol 120, Feb 2002.
 A. Schett, C.R. Keeler, The Ophthalmoscope. P. 81.